diff --git a/src/compiler/sys.ts b/src/compiler/sys.ts
index be0d973032e..32d7af09c03 100644
--- a/src/compiler/sys.ts
+++ b/src/compiler/sys.ts
@@ -1445,6 +1445,7 @@ export interface System {
     realpath?(path: string): string;
     /** @internal */ getEnvironmentVariable(name: string): string;
     /** @internal */ tryEnableSourceMapsForHost?(): void;
+    /** @internal */ getAccessibleFileSystemEntries?(path: string): FileSystemEntries;
     /** @internal */ debugMode?: boolean;
     setTimeout?(callback: (...args: any[]) => void, ms: number, ...args: any[]): any;
     clearTimeout?(timeoutId: any): void;
@@ -1553,6 +1554,7 @@ export let sys: System = (() => {
             resolvePath: path => _path.resolve(path),
             fileExists,
             directoryExists,
+            getAccessibleFileSystemEntries,
             createDirectory(directoryName: string) {
                 if (!nodeSystem.directoryExists(directoryName)) {
                     // Wrapped in a try-catch to prevent crashing if we are in a race
diff --git a/src/harness/harnessIO.ts b/src/harness/harnessIO.ts
index 298d8310161..7b7506d30f5 100644
--- a/src/harness/harnessIO.ts
+++ b/src/harness/harnessIO.ts
@@ -25,7 +25,7 @@ export interface IO {
     fileExists(fileName: string): boolean;
     directoryExists(path: string): boolean;
     deleteFile(fileName: string): void;
-    enumerateTestFiles(runner: RunnerBase): (string | FileBasedTest)[];
+    enumerateTestFiles(runner: RunnerBase): string[];
     listFiles(path: string, filter?: RegExp, options?: { recursive?: boolean; }): string[];
     log(text: string): void;
     args(): string[];
@@ -84,55 +84,28 @@ function createNodeIO(): IO {
         return runner.getTestFiles();
     }
 
-    function listFiles(path: string, spec: RegExp, options: { recursive?: boolean; } = {}) {
+    function listFiles(path: string, spec: RegExp | undefined, options: { recursive?: boolean; } = {}) {
         function filesInFolder(folder: string): string[] {
+            const { files, directories } = ts.sys.getAccessibleFileSystemEntries!(folder);
             let paths: string[] = [];
-
-            for (const file of fs.readdirSync(folder)) {
+            for (const file of files) {
                 const pathToFile = pathModule.join(folder, file);
-                if (!fs.existsSync(pathToFile)) continue; // ignore invalid symlinks
-                const stat = fs.statSync(pathToFile);
-                if (options.recursive && stat.isDirectory()) {
-                    paths = paths.concat(filesInFolder(pathToFile));
-                }
-                else if (stat.isFile() && (!spec || file.match(spec))) {
+                if (!spec || file.match(spec)) {
                     paths.push(pathToFile);
                 }
             }
-
+            if (options.recursive) {
+                for (const dir of directories) {
+                    const pathToDir = pathModule.join(folder, dir);
+                    paths = paths.concat(filesInFolder(pathToDir));
+                }
+            }
             return paths;
         }
 
         return filesInFolder(path);
     }
 
-    function getAccessibleFileSystemEntries(dirname: string): ts.FileSystemEntries {
-        try {
-            const entries: string[] = fs.readdirSync(dirname || ".").sort(ts.sys.useCaseSensitiveFileNames ? ts.compareStringsCaseSensitive : ts.compareStringsCaseInsensitive);
-            const files: string[] = [];
-            const directories: string[] = [];
-            for (const entry of entries) {
-                if (entry === "." || entry === "..") continue;
-                const name = vpath.combine(dirname, entry);
-                try {
-                    const stat = fs.statSync(name);
-                    if (!stat) continue;
-                    if (stat.isFile()) {
-                        files.push(entry);
-                    }
-                    else if (stat.isDirectory()) {
-                        directories.push(entry);
-                    }
-                }
-                catch { /*ignore*/ }
-            }
-            return { files, directories };
-        }
-        catch (e) {
-            return { files: [], directories: [] };
-        }
-    }
-
     function createDirectory(path: string) {
         try {
             fs.mkdirSync(path);
@@ -170,7 +143,7 @@ function createNodeIO(): IO {
         getWorkspaceRoot: () => workspaceRoot,
         exit: exitCode => ts.sys.exit(exitCode),
         readDirectory: (path, extension, exclude, include, depth) => ts.sys.readDirectory(path, extension, exclude, include, depth),
-        getAccessibleFileSystemEntries,
+        getAccessibleFileSystemEntries: ts.sys.getAccessibleFileSystemEntries!,
         tryEnableSourceMapsForHost: () => ts.sys.tryEnableSourceMapsForHost && ts.sys.tryEnableSourceMapsForHost(),
         getMemoryUsage: () => ts.sys.getMemoryUsage && ts.sys.getMemoryUsage(),
         getEnvironmentVariable: name => ts.sys.getEnvironmentVariable(name),
diff --git a/src/harness/runnerbase.ts b/src/harness/runnerbase.ts
index 1877d763e9f..fbe8f32b299 100644
--- a/src/harness/runnerbase.ts
+++ b/src/harness/runnerbase.ts
@@ -1,5 +1,4 @@
 import {
-    FileBasedTest,
     IO,
     userSpecifiedRoot,
 } from "./_namespaces/Harness";
@@ -22,7 +21,7 @@ export function setShardId(id: number) {
 
 export abstract class RunnerBase {
     // contains the tests to run
-    public tests: (string | FileBasedTest)[] = [];
+    public tests: string[] = [];
 
     /** Add a source file to the runner's list of tests that need to be initialized with initializeTests */
     public addTest(fileName: string) {
@@ -35,7 +34,7 @@ export abstract class RunnerBase {
 
     abstract kind(): TestRunnerKind;
 
-    abstract enumerateTestFiles(): (string | FileBasedTest)[];
+    abstract enumerateTestFiles(): string[];
 
     getTestFiles(): ReturnType<this["enumerateTestFiles"]> {
         const all = this.enumerateTestFiles();
diff --git a/src/testRunner/compilerRunner.ts b/src/testRunner/compilerRunner.ts
index b3c86f6390f..b5d08ca8f2e 100644
--- a/src/testRunner/compilerRunner.ts
+++ b/src/testRunner/compilerRunner.ts
@@ -51,9 +51,10 @@ export class CompilerBaselineRunner extends RunnerBase {
         return this.testSuiteName;
     }
 
+    private testFiles: string[] | undefined;
     public enumerateTestFiles() {
         // see also: `enumerateTestFiles` in tests/webTestServer.ts
-        return this.enumerateFiles(this.basePath, /\.tsx?$/, { recursive: true }).map(CompilerTest.getConfigurations);
+        return this.testFiles ??= this.enumerateFiles(this.basePath, /\.tsx?$/, { recursive: true });
     }
 
     public initializeTests() {
@@ -64,9 +65,8 @@ export class CompilerBaselineRunner extends RunnerBase {
 
             // this will set up a series of describe/it blocks to run between the setup and cleanup phases
             const files = this.tests.length > 0 ? this.tests : IO.enumerateTestFiles(this);
-            files.forEach(test => {
-                const file = typeof test === "string" ? test : test.file;
-                this.checkTestCodeOutput(vpath.normalizeSeparators(file), typeof test === "string" ? CompilerTest.getConfigurations(test) : test);
+            files.forEach(file => {
+                this.checkTestCodeOutput(vpath.normalizeSeparators(file), CompilerTest.getConfigurations(file));
             });
         });
     }
diff --git a/src/testRunner/fourslashRunner.ts b/src/testRunner/fourslashRunner.ts
index 689cdcceca9..6fd61eb0e3b 100644
--- a/src/testRunner/fourslashRunner.ts
+++ b/src/testRunner/fourslashRunner.ts
@@ -41,8 +41,7 @@ export class FourSlashRunner extends RunnerBase {
         }
 
         describe(this.testSuiteName + " tests", () => {
-            this.tests.forEach(test => {
-                const file = typeof test === "string" ? test : test.file;
+            this.tests.forEach(file => {
                 describe(file, () => {
                     let fn = ts.normalizeSlashes(file);
                     const justName = fn.replace(/^.*[\\/]/, "");
diff --git a/src/testRunner/parallel/host.ts b/src/testRunner/parallel/host.ts
index 126e06cff4e..ca8ffb563d3 100644
--- a/src/testRunner/parallel/host.ts
+++ b/src/testRunner/parallel/host.ts
@@ -217,8 +217,7 @@ export function start() {
         console.log("Discovering runner-based tests...");
         const discoverStart = +(new Date());
         for (const runner of runners) {
-            for (const test of runner.getTestFiles()) {
-                const file = typeof test === "string" ? test : test.file;
+            for (const file of runner.getTestFiles()) {
                 let size: number;
                 if (!perfData) {
                     try {
diff --git a/src/testRunner/projectsRunner.ts b/src/testRunner/projectsRunner.ts
index dd2a2f40f6e..bedb3343e7c 100644
--- a/src/testRunner/projectsRunner.ts
+++ b/src/testRunner/projectsRunner.ts
@@ -54,7 +54,7 @@ export class ProjectRunner extends Harness.RunnerBase {
         describe("projects tests", () => {
             const tests = this.tests.length === 0 ? this.enumerateTestFiles() : this.tests;
             for (const test of tests) {
-                this.runProjectTestCase(typeof test === "string" ? test : test.file);
+                this.runProjectTestCase(test);
             }
         });
     }
@@ -202,15 +202,36 @@ class ProjectTestCase {
             throw assert(false, "Testcase: " + testCaseFileName + " does not contain valid json format: " + e.message);
         }
 
-        const fs = vfs.createFromFileSystem(Harness.IO, /*ignoreCase*/ false);
-        fs.mountSync(vpath.resolve(Harness.IO.getWorkspaceRoot(), "tests"), vpath.combine(vfs.srcFolder, "tests"), vfs.createResolver(Harness.IO));
-        fs.mkdirpSync(vpath.combine(vfs.srcFolder, testCase.projectRoot));
-        fs.chdir(vpath.combine(vfs.srcFolder, testCase.projectRoot));
-        fs.makeReadonly();
-
+        function makeFileSystem() {
+            const fs = vfs.createFromFileSystem(Harness.IO, /*ignoreCase*/ false);
+            fs.mountSync(vpath.resolve(Harness.IO.getWorkspaceRoot(), "tests"), vpath.combine(vfs.srcFolder, "tests"), vfs.createResolver(Harness.IO));
+            fs.mkdirpSync(vpath.combine(vfs.srcFolder, testCase.projectRoot));
+            fs.chdir(vpath.combine(vfs.srcFolder, testCase.projectRoot));
+            fs.makeReadonly();
+            return fs;
+        }
+        let fs: vfs.FileSystem | undefined;
         return [
-            { name: `@module: commonjs`, payload: { testCase, moduleKind: ts.ModuleKind.CommonJS, vfs: fs } },
-            { name: `@module: amd`, payload: { testCase, moduleKind: ts.ModuleKind.AMD, vfs: fs } },
+            {
+                name: `@module: commonjs`,
+                payload: {
+                    testCase,
+                    moduleKind: ts.ModuleKind.CommonJS,
+                    get vfs() {
+                        return fs ??= makeFileSystem();
+                    },
+                },
+            },
+            {
+                name: `@module: amd`,
+                payload: {
+                    testCase,
+                    moduleKind: ts.ModuleKind.AMD,
+                    get vfs() {
+                        return fs ??= makeFileSystem();
+                    },
+                },
+            },
         ];
     }
 
diff --git a/src/testRunner/runner.ts b/src/testRunner/runner.ts
index d8b7bbed8a5..ecf720996e4 100644
--- a/src/testRunner/runner.ts
+++ b/src/testRunner/runner.ts
@@ -27,8 +27,7 @@ function runTests(runners: RunnerBase[]) {
         const dupes: [string, string][] = [];
         for (const runner of runners) {
             if (runner instanceof CompilerBaselineRunner || runner instanceof FourSlashRunner) {
-                for (const sf of runner.enumerateTestFiles()) {
-                    const full = typeof sf === "string" ? sf : sf.file;
+                for (const full of runner.enumerateTestFiles()) {
                     const base = vpath.basename(full).toLowerCase();
                     // allow existing dupes in fourslash/shims and fourslash/server
                     if (seen.has(base) && !/fourslash\/(shim|server)/.test(full)) {
@@ -191,7 +190,6 @@ function handleTestConfig() {
                     case "fourslash-generated":
                         runners.push(new GeneratedFourslashRunner(FourSlash.FourSlashTestType.Native));
                         break;
-                        break;
                 }
             }
         }