diff --git a/groupby.go b/groupby.go
index e4a554b..7b5db2d 100644
--- a/groupby.go
+++ b/groupby.go
@@ -2,7 +2,7 @@ package underscore
 
 // GroupBy splits a slice into a map[K][]V grouped by the result of the iterator function.
 func GroupBy[K comparable, V any](values []V, f func(V) K) map[K][]V {
-	res := make(map[K][]V, 0)
+	res := make(map[K][]V, len(values)/10)
 	for _, v := range values {
 		k := f(v)
 		if r, ok := res[k]; ok {