I'm trying to find a video player for KDE that will let me slow down or speed up videos while I'm watching them. So far, none of Dragon Player, Kaffeine, KPlayer, KMPlayer, and Bangarang seem to have this feature. Could somebody point me in the right direction?

They don't have it because those are just players, not editors, changing the speed is something you mostly use in video editing. Try having a look at Kdenlive instead.

try sMplayer (QT based) or VLC

I didn't want to permanently change the playback speed, and this is a feature that even Windows Media Player has, so I thought it would be a lot easier to find an alternative here without resorting to VLC. SMPlayer does it (though it's also pretty ugly), so I'll be trying both that and VLC. Thanks!

you could also look at umPlayer, a fork of sMplayer iirc