While Kdenlive already supprts using multiple threads for timeline preview in principle, there are multiple reports that Kdenlive then becomes unusable on certain fast systems, such as Core i7 machines.

In particular, there seems to be some race conditions that cause preview rendering of a single frame to complete faster than the UI thread triggering the render operation. In consequence, update of the playhead doesn't work anymore: when clicking on a position in the timeline ruler, the small ruler mark moves to this new position, but the playhead never catches up. In turn, it's impossible to edit in the timeline, because the playhead doesn't correctly reflect its real position. When playing back the timeline, playhead operation works as expected though.