gofer: reword perf warning to match perf warning regex

test/runner/main.go has `performanceWarningRegexp` set as:

```
var performanceWarningRegexp =
    regexp.MustCompile(`(?i).*(slows?|slowing).*(startup|teardown).*`)
```

the existing warning did not match this regex, causing flaky readv tests in
buildkite.

This should fix it.

PiperOrigin-RevId: 970157268