I mentioned in another thread about lag when using shift, I think it is actually this issue. If the line I try to draw is too short it just won't draw, which feels like lag. Perhaps you're right about it being unable to work out what I want to do, i.e. snap to horizontal, vertical or diagonal.
I'm a product designer so I draw a lot of sections of how stuff fits together, hence I use constrain to horizontal/vertical a lot. It's a bit of nuisance but I'm sure I'll work around it.
I haven't tried SBPro to see if it does the same thing, might try it although I prefer working in Painter.
Thanks for your help (again), JoeC.