diff --git a/lib/fs/util.go b/lib/fs/util.go
index 348124dcf..2ffd6cc22 100644
--- a/lib/fs/util.go
+++ b/lib/fs/util.go
@@ -47,25 +47,13 @@ func getHomeDir() (string, error) {
 	return os.UserHomeDir()
 }
 
-var (
-	windowsDisallowedCharacters = string([]rune{
-		'<', '>', ':', '"', '|', '?', '*',
-		0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
-		11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
-		21, 22, 23, 24, 25, 26, 27, 28, 29, 30,
-		31,
-	})
-	windowsDisallowedNames = []string{"CON", "PRN", "AUX", "NUL",
-		"COM0", "COM1", "COM2", "COM3", "COM4", "COM5", "COM6", "COM7", "COM8", "COM9",
-		"LPT0", "LPT1", "LPT2", "LPT3", "LPT4", "LPT5", "LPT6", "LPT7", "LPT8", "LPT9",
-	}
-)
+const windowsDisallowedCharacters = (`<>:"|?*` +
+	"\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f" +
+	"\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f")
 
 func WindowsInvalidFilename(name string) error {
 	// None of the path components should end in space or period, or be a
-	// reserved name. COM0 and LPT0 are missing from the Microsoft docs,
-	// but Windows Explorer treats them as invalid too.
-	// (https://docs.microsoft.com/windows/win32/fileio/naming-a-file)
+	// reserved name.
 	for _, part := range strings.Split(name, `\`) {
 		if len(part) == 0 {
 			continue
@@ -110,7 +98,7 @@ func WindowsInvalidFilename(name string) error {
 func SanitizePath(path string) string {
 	var b strings.Builder
 
-	disallowed := `<>:"'/\|?*[]{};:!@$%&^#` + windowsDisallowedCharacters
+	const disallowed = `'/\[]{};:!@$%&^#` + windowsDisallowedCharacters
 	prev := ' '
 	for _, c := range path {
 		if !unicode.IsPrint(c) || c == unicode.ReplacementChar ||
@@ -132,15 +120,27 @@ func SanitizePath(path string) string {
 }
 
 func windowsIsReserved(part string) bool {
-	upperCased := strings.ToUpper(part)
-	for _, disallowed := range windowsDisallowedNames {
-		if upperCased == disallowed {
-			return true
-		}
-		if strings.HasPrefix(upperCased, disallowed+".") {
-			// nul.txt.jpg is also disallowed
-			return true
-		}
+	// nul.txt.jpg is also disallowed.
+	dot := strings.IndexByte(part, '.')
+	if dot != -1 {
+		part = part[:dot]
+	}
+
+	// Check length to skip allocating ToUpper.
+	if len(part) != 3 && len(part) != 4 {
+		return false
+	}
+
+	// COM0 and LPT0 are missing from the Microsoft docs,
+	// but Windows Explorer treats them as invalid too.
+	// (https://docs.microsoft.com/windows/win32/fileio/naming-a-file)
+	switch strings.ToUpper(part) {
+	case "CON", "PRN", "AUX", "NUL",
+		"COM0", "COM1", "COM2", "COM3", "COM4",
+		"COM5", "COM6", "COM7", "COM8", "COM9",
+		"LPT0", "LPT1", "LPT2", "LPT3", "LPT4",
+		"LPT5", "LPT6", "LPT7", "LPT8", "LPT9":
+		return true
 	}
 	return false
 }
diff --git a/lib/fs/util_test.go b/lib/fs/util_test.go
index 0b934308a..a263b5ef9 100644
--- a/lib/fs/util_test.go
+++ b/lib/fs/util_test.go
@@ -117,3 +117,15 @@ func TestSanitizePathFuzz(t *testing.T) {
 		}
 	}
 }
+
+func benchmarkWindowsInvalidFilename(b *testing.B, name string) {
+	for i := 0; i < b.N; i++ {
+		WindowsInvalidFilename(name)
+	}
+}
+func BenchmarkWindowsInvalidFilenameValid(b *testing.B) {
+	benchmarkWindowsInvalidFilename(b, "License.txt.gz")
+}
+func BenchmarkWindowsInvalidFilenameNUL(b *testing.B) {
+	benchmarkWindowsInvalidFilename(b, "nul.txt.gz")
+}