I think there may be a possibility of the terminal itself being slow. Years ago when I used semi transparent terminals in something like enlightenment it was seriously noticeable when you were trying to scroll through text and it'd take a while to appear. Then I switched to using a simple terminal like rxvt and no transparency effects under ion2 and wow was it fast.

So, perhaps the answer is that it's not the editor nor plugins that's slow in the scenario. I think the line numbering even causes some slight slowdown. Anything that actually parses text will obviously cause a performance hit too. I notice slowdowns when I use visual selection modes combined with tricky combos of commands. Even without any of that, it might be possible that the version of vim is doing something stupid like what happens when you open up a 1GB log file in less and hit G. Haven't run an strafe to understand the fseek calls being made but that's been painful without exception in my experience. Perhaps that's what the poster was recalling.