Go agouti testing fill textarea with PhantomJS perfomance issue

I am using agouti with gomega and ginkgo in Go to test an upload form of our application consisting of an textarea which we fill.

This Code works fine for 1500 rows:

upload_externalData := page.Find("#upload_externalData")
buf := bytes.NewBuffer(nil)
f, err := os.Open("./files/external.log")
io.Copy(buf, f)
externalData := string(buf.Bytes())
Expect(upload_externalData.Fill(string(externalData))).Should(Succeed())

When increasing the imported data to the normal 25000 rows, PhantomJS blocks one CPU Core at 100% and nothing else happens.

Is there a way to achieve this?