That's arguably part of the solution/problem. It's trial and error until you find one that works, and has a decent interface you can deal with. Also, original files tend to vary, whether or not they are from the internet or created by you.
FWIW, you might look into Handbrake for your converting and/or transcoding.
To avoid some of the pain of dealing with a slow laptop you could grab a media player such as a WDTV Live, which can play back nearly everything you throw at it.