I took GRE without prep as a non-native speaker. Lets just say I severely underestimated it, and went on to do other things in life … the concept of these tests are quite foreign in Northern Europe, so I was not expecting it /at all/. If I wanted to ace it, it would've probably taken me weeks to prep properly (especially all the silly math things, that I mostly forgot about how to do fast enough without mistakes or stressing out). Never ended up submitting my application, even though I had several first author papers as an undergrad, I felt too defeated by it.