i don't think so. the gmat is not trying to test out your abilities in computing long math by hand. if something is taking you more than 2 minutes, that means you've missed a shortcut or number property to allow you do the problem quickly OR it's an experimental question. either way, you should move on if the problem is taking too much of your time. if you memorize common square roots and cube roots, you'll be fine.
post558169.html#558169 - a compilation of b-school interview questions
8-66345.html - my (detailed) gmat debrief