You should do questions in batches. One at a time has no bearing on the GMAT. Some questions naturally take longer and others do not.

For example, the last time I took the actual GMAT I looked to be a few minutes behind on the Quant, then I got 5 Data Sufficiency in a row and was right back on pace.

Sets of 10 are about right with an overall time goal - knowing that not every question will be two minutes.

Here is an article that I wrote about a different way of doing some sets that can actually help your sense of timing on test day. Remember that on test day you do not want to look at the clock very often. Pace is something that has to happen naturally by the time you get to test day. https://www.beatthegmat.com/mba/2012/04/ ... at-success