diff --git a/lib/fs/folding.go b/lib/fs/folding.go
index 20887b95e..1a8791d43 100644
--- a/lib/fs/folding.go
+++ b/lib/fs/folding.go
@@ -17,6 +17,52 @@ import (
 // UnicodeLowercaseNormalized returns the Unicode lower case variant of s,
 // having also normalized it to normalization form C.
 func UnicodeLowercaseNormalized(s string) string {
+	if isASCII, isLower := isASCII(s); isASCII {
+		if isLower {
+			return s
+		}
+		return toLowerASCII(s)
+	}
+
+	return toLowerUnicode(s)
+}
+
+func isASCII(s string) (bool, bool) {
+	isLower := true
+	for _, b := range []byte(s) {
+		if b > unicode.MaxASCII {
+			return false, isLower
+		}
+		if 'A' <= b && b <= 'Z' {
+			isLower = false
+		}
+	}
+	return true, isLower
+}
+
+func toLowerASCII(s string) string {
+	var (
+		b   strings.Builder
+		pos int
+	)
+	b.Grow(len(s))
+	for i, c := range []byte(s) {
+		if c < 'A' || 'Z' < c {
+			continue
+		}
+		if pos < i {
+			b.WriteString(s[pos:i])
+		}
+		pos = i + 1
+		b.WriteByte(c + 'a' - 'A')
+	}
+	if pos != len(s) {
+		b.WriteString(s[pos:])
+	}
+	return b.String()
+}
+
+func toLowerUnicode(s string) string {
 	i := firstCaseChange(s)
 	if i == -1 {
 		return norm.NFC.String(s)
@@ -30,7 +76,11 @@ func UnicodeLowercaseNormalized(s string) string {
 	rs.WriteString(s[:i])
 
 	for _, r := range s[i:] {
-		rs.WriteRune(unicode.ToLower(unicode.ToUpper(r)))
+		if r <= unicode.MaxLatin1 && r != 'µ' {
+			rs.WriteRune(unicode.ToLower(r))
+		} else {
+			rs.WriteRune(unicode.To(unicode.LowerCase, unicode.To(unicode.UpperCase, r)))
+		}
 	}
 	return norm.NFC.String(rs.String())
 }
@@ -38,10 +88,13 @@ func UnicodeLowercaseNormalized(s string) string {
 // Byte index of the first rune r s.t. lower(upper(r)) != r.
 func firstCaseChange(s string) int {
 	for i, r := range s {
-		if r <= unicode.MaxASCII && (r < 'A' || r > 'Z') {
-			continue
+		if r <= unicode.MaxASCII {
+			if r < 'A' || r > 'Z' {
+				continue
+			}
+			return i
 		}
-		if unicode.ToLower(unicode.ToUpper(r)) != r {
+		if unicode.To(unicode.LowerCase, unicode.To(unicode.UpperCase, r)) != r {
 			return i
 		}
 	}
diff --git a/lib/fs/folding_test.go b/lib/fs/folding_test.go
index c3e05fa8b..fd944047b 100644
--- a/lib/fs/folding_test.go
+++ b/lib/fs/folding_test.go
@@ -49,6 +49,18 @@ var caseCases = [][2]string{
 	{"a\xCC\x88", "\xC3\xA4"}, // ä
 }
 
+var benchmarkCases = [][2]string{
+	{"img_202401241010.jpg", "ASCII lowercase"},
+	{"IMG_202401241010.jpg", "ASCII mixedcase start"},
+	{"img_202401241010.JPG", "ASCII mixedcase end"},
+	{"wir_kinder_aus_bullerbü.epub", "Latin1 lowercase"},
+	{"Wir_Kinder_aus_Bullerbü.epub", "Latin1 mixedcase start"},
+	{"wir_kinder_aus_bullerbü.EPUB", "Latin1 mixedcase end"},
+	{"translated_ウェブの国際化.html", "Unicode lowercase"},
+	{"Translated_ウェブの国際化.html", "Unicode mixedcase start"},
+	{"translated_ウェブの国際化.HTML", "Unicode mixedcase end"},
+}
+
 func TestUnicodeLowercaseNormalized(t *testing.T) {
 	for _, tc := range caseCases {
 		res := UnicodeLowercaseNormalized(tc[0])
@@ -58,22 +70,13 @@ func TestUnicodeLowercaseNormalized(t *testing.T) {
 	}
 }
 
-func BenchmarkUnicodeLowercaseMaybeChange(b *testing.B) {
-	b.ReportAllocs()
-
-	for i := 0; i < b.N; i++ {
-		for _, s := range caseCases {
-			UnicodeLowercaseNormalized(s[0])
-		}
-	}
-}
-
-func BenchmarkUnicodeLowercaseNoChange(b *testing.B) {
-	b.ReportAllocs()
-
-	for i := 0; i < b.N; i++ {
-		for _, s := range caseCases {
-			UnicodeLowercaseNormalized(s[1])
-		}
+func BenchmarkUnicodeLowercase(b *testing.B) {
+	for _, c := range benchmarkCases {
+		b.Run(c[1], func(b *testing.B) {
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				UnicodeLowercaseNormalized(c[0])
+			}
+		})
 	}
 }