ci: explain why ci runs tests in serial now

ci/slow.go

@@ -16,6 +16,9 @@ func SkipSlow(t *testing.T, reason string) {
 }
 
 // Parallel runs t in parallel, unless CI is set to a true value.
+//
+// In CI (CircleCI / GitHub Actions) we get better performance by running tests
+// in serial while not restricting GOMAXPROCS.
 func Parallel(t *testing.T) {
 	value := os.Getenv("CI")
 	isCI, err := strconv.ParseBool(value)