On SciTE 3.3.5 if a very long line (for example 1 million characters or more) is in the visible area the performance will decrease heavily. Any input actions will be delayed for ~1 second.
But as soon as this line gets out of the visible area all is fine again. So it seems SciTE is making some internal optimizations to process only the visible lines. I'm wondering if this could also be done in a horizontal way: All content of a line which is beyond the horizontal scrollbar does get this sort of optimization too.