Timed online assessments (HackerRank, CodeSignal) wreck me way more than the actual onsite

I can solve these problems fine when I'm relaxed. Put a 70-minute countdown on the screen and my brain just... leaves. Last one I misread the constraints, burned 25 minutes on the wrong approach, and never recovered. Onsites where I can actually talk to a human go so much better.

Is it just reps until the timer stops mattering, or are there tricks? Do you read all the questions first, or start immediately on #1?