diff --git a/.changeset/poor-sites-buy.md b/.changeset/poor-sites-buy.md
new file mode 100644
index 0000000..59591f7
--- /dev/null
+++ b/.changeset/poor-sites-buy.md
@@ -0,0 +1,5 @@
+---
+'vite-plugin-static-copy': patch
+---
+
+improve performance of internal `isSubdirectoryOrEqual` function
diff --git a/src/utils.test.ts b/src/utils.test.ts
index 7a647f7..f0d6e60 100644
--- a/src/utils.test.ts
+++ b/src/utils.test.ts
@@ -1,6 +1,43 @@
 import { describe, expect, test } from 'vitest'
-import { groupTargetsByDirectoryTree } from './utils'
+import { isSubdirectoryOrEqual, groupTargetsByDirectoryTree } from './utils'
 import path from 'node:path'
+import { fileURLToPath } from 'node:url'
+import os from 'node:os'
+
+const _dirname = path.dirname(fileURLToPath(import.meta.url))
+const isWindows = os.platform() === 'win32'
+
+describe('isSubdirectoryOrEqual', () => {
+  const cases: readonly [a: string, b: string, expected: boolean][] = [
+    ['./', '.', true],
+    ['./', './', true],
+    ['.', './', true],
+    ['./index.ts', './', true],
+    ['./foo/', './', true],
+    ['./foo/bar', './', true],
+    ['./foo/bar.js', './', true],
+    ['..', './', false],
+    ['../', './', false],
+    ['../test', './', false],
+    ['../test/', './', false],
+    ...(isWindows
+      ? ([
+          ['C:/', 'C:/', true],
+          ['C:\\', 'C:/', true],
+          ['C:/', 'D:/', false],
+          ['C:\\', 'D:/', false]
+        ] satisfies readonly [string, string, boolean][])
+      : [])
+  ]
+
+  const resolve = (p: string) => path.resolve(_dirname, p)
+
+  for (const [a, b, expected] of cases) {
+    test(`isSubdirectoryOrEqual(${a}, ${b})`, () => {
+      expect(isSubdirectoryOrEqual(resolve(a), resolve(b))).toBe(expected)
+    })
+  }
+})
 
 describe('groupTargetsByDirectoryTree', () => {
   const defineCase = (input: string[], expected: string[][]) => ({
diff --git a/src/utils.ts b/src/utils.ts
index 0d69c5e..aa5163f 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -27,12 +27,14 @@ type ResolvedTarget = SimpleTarget & {
   resolvedSrc: string
 }
 
-const isSubdirectoryOrEqual = (a: string, b: string) => {
-  const relative = path.relative(b, a)
-  return (
-    !relative ||
-    (!relative.startsWith(`..${path.sep}`) && !path.isAbsolute(relative))
-  )
+/**
+ * Whether a is a subdirectory of b or equal to b
+ *
+ * @param a absolute path
+ * @param b absolute path
+ */
+export const isSubdirectoryOrEqual = (a: string, b: string) => {
+  return a.startsWith(b + path.sep) || a === b
 }
 
 export const groupTargetsByDirectoryTree = <T extends { resolvedDest: string }>(