There are two separate issues at play here: Emacs' redisplay algorithm is not very efficient if you have very long lines. On the other had on old emacs version

There are two separate issues at play here: Emacs' redisplay algorithm is not very efficient if you have very long lines. On the other had on old emacs versions (before 23 IIRC) this was significantly worse as it also had weird UX implications (C-p/C-n working in terms of logical, as opposed to displayed, lines and unability to sanely display lines that are longer than width*height of window). Other problem is that the gap-buffer data structure is very suboptimal when you start to move the point by multi-MB offsets (which in emacs' case imcludes operations like scrolling to the other end of buffer using scrollbar) as it involves doing memmove() of this multi-MB block of memory.