I recommend an 0.7 mm or even 0.9 mm pencil. The 0.5 one, if you're writing fast and hard, can tear the test paper, as I learned when I took the June test. The paper on the test is nothing like the paper on the preptests; it's flimsy and thin.
I used a 0.5 and encountered no problems... but none of this stuff even occurred to me beforehand. I wouldn't stress too much over small details like this. In fact my proctor even had pencils on hand for students, just your typical yellow no. 2 things... I remember cause I was pretty surprised by how many people took her up on the offer. The truth is most people simply are not prepared for the LSAT and haven't put much effort into studying or anything. So that puts those of us who care and prepare in a better position
Actually, since the lead is thicker, I thought it'd take less time. But as Letylyf said, these are minor details. I found that with my 0.5 mm pencil, I tore the test paper a couple of times, but that might say more about me than about the test paper!
I agree with your point about people being unprepared. I think this point is true for all aspects of life. It is amazing how good you can look by just doing simple things, because most people don't even do those things.