diff --git a/src/strings/strings.go b/src/strings/strings.go
index 5f244d6e20..88fbeecc6f 100644
--- a/src/strings/strings.go
+++ b/src/strings/strings.go
@@ -379,25 +379,29 @@ func FieldsFunc(s string, f func(rune) bool) []string {
 	spans := make([]span, 0, 32)
 
 	// Find the field start and end indices.
-	wasField := false
-	fromIndex := 0
-	for i, rune := range s {
+	// Doing this in a separate pass (rather than slicing the string s
+	// and collecting the result substrings right away) is significantly
+	// more efficient, possibly due to cache effects.
+	start := -1 // valid span start if >= 0
+	for end, rune := range s {
 		if f(rune) {
-			if wasField {
-				spans = append(spans, span{start: fromIndex, end: i})
-				wasField = false
+			if start >= 0 {
+				spans = append(spans, span{start, end})
+				// Set start to a negative value.
+				// Note: using -1 here consistently and reproducibly
+				// slows down this code by a several percent on amd64.
+				start = ^start
 			}
 		} else {
-			if !wasField {
-				fromIndex = i
-				wasField = true
+			if start < 0 {
+				start = end
 			}
 		}
 	}
 
 	// Last field might end at EOF.
-	if wasField {
-		spans = append(spans, span{fromIndex, len(s)})
+	if start >= 0 {
+		spans = append(spans, span{start, len(s)})
 	}
 
 	// Create strings from recorded field indices.