And horizontal scrolling is not laggy with touchpad at all. Well, at least rapid left and right swipes on touchpad aren't creating enormous cpu usage.
So, to summarize:
- scrolling to buffer end (down) is less laggy than scrolling to buffer start (up)
- horizontal scrolling isn't laggy at all
- this can be achieved with mouse if you roll it with mouse wheel over the table very fast
If I can produce more information, maybe stack traces, or more in-depth profiling please guide me with some instructions. This issue bothered me for about 2 years, just finally decided to report it.