Welcome to GMAT club and thank you for the question.
First of all, the Manhattan tests are definitely more extensive when it gets to quant. And laborious is a good word for it. Sometimes you cannot finish the calculations in less than two or three minutes and that's kind of annoying.
As it relates to sitting still, I really don't remember having issues with that a whole lot, mostly because it was so freaked out by the timer and by every second I was losing thinking about anything other than the question.
On the test day, there were a lot of distractions of the test center anything from fluorescent lights making buzzing noise to a neighbor of mine typing away a war and peace on their computer. They were taking a different test and or on a different timeline - I definitely make use of the ear plugs. You will definitely have to cancel out a lot of the distractions and be fully immersed in the test. When I was done with quantum and it was time to take a break I glanced to my right and saw that my neighbor was already gone. I didn't even notice when he got up and stopes typing away.
How are you doing on timing with your questions? Are you finding that you're running low on time or you feel like you have time left over? Are you also timing yourself when doing OG? There's a lot to be said about the test stamina and practicing the entire three and a half hour ordeal. It's super painful but it teaches you how to conserve your strength and gets easier over time.
Sent from my iPhone using
GMAT Club Forum App