That extra 50mA may be required for the signal conversion circuitry.
No such thing. The only difference is a crystal on the board which is rated at a different clock speed, so that the video output will be 50Hz instead of 60 (that's all it takes for PAL output - no need for "signal conversion"). In other words, this is the exact same hardware, with the exception of that single crystal.
Also, slowdown is a misleading term when talking about PAL-50 output... Because while games generally run somewhat slower (constantly, not just occasionally, which slowdown means), the frame rates are actually higher. It's a little hard to explain, but think about ACTUAL slowdown in games... like, when a game starts running at maybe 30% speed because there's too much stuff going on at the screen... well, in PAL mode, these slowdowns rarely occur.
It's pretty fun to play a game console with a 50/60 Hz switch... whenever there's slowdown in a game, I just flip it to 50Hz and it all runs faster right away.
I don't know what causes it, but I think that Bonknuts briefly explained this somewhere once.