You have to go slow & careful at first, but once you get it down, I can square file sharpen just as fast as round (by hand).
I have developed a routine. I use a stump vice on the bar, but it on a wooden box (that I haul my chainsaw stuff in) and sit near it on a stool (intended to do car breaks). This puts the bar lower than I would for round filing. I do one side at a time, and move each tooth to the same place to keep the angles consistent.
I have 45 degree lines drawn on the top of the box to help me keep the angles correct (or I find myself reverting to the old 30 degrees). When the tooth is where I want it, I put a paint stirrer behind it and pinch the stirrer to the bar with my off hand and sharpen the tooth with the other hand (keeps the tooth from moving, and saves you from putting the chain break on & off a million times). It seems to work well for me.
Remember, with square file you go from outside in (instead of inside out), and the file is really 6 sided, not square. And don't be concerned if you touch the straps on the opposite side, you will.