MPlayer has always supported these configuration options.

The point is that XV got ARMv6 optimizations for the color format conversion (https://bugs.maemo.org/show_bug.cgi?id=1278), but also now supports colorkey with the latest diablo update.

And XV is fast not in all cases on N8x0 devices. There are quite a number of cases when it can fallback to a slow implementation. For the example of unfavourable placement one can see https://bugs.maemo.org/show_bug.cgi?id=1281

Using a colorkey (and mplayer tries to mess with colorkey by default) also slows everything down. Having something displayed over the video overlay (poweroff menu or audio volume settings dialog, or whatever) also hurts performance.