There are a few mistakes in your aproach:

- Where you boot from has little to no impact to video playback performance - it may have impact to boot speed and program load times but once the video player app is loaded the cpu is the bottleneck not the disk access speed. BTW your videos are probably always on a sd card so it really doesn't matter where you boot from, the sd card speed will always be the same.
- File size says nothing about VIDEO size. You might have a 10 mb file at a high bitrate and/or resolution - which chokes the tablet, also you might have a 1 gb file at a low resolution which the tablet will play just fine. Also the codec used for the videos has a great impact on performance.
- If you want to do some serious video playback I sugest you install & use MPlayer (search the forum, you'll find it and you can install it from the maemo extras repositories). You'll find that some codecs work better than others - again search this forum. Also transcoding your files to something the tablet likes will do wonders. Again search. Bottom line MPlayer is a lot faster than the built in video player. I find that many of my video files don't require transcoding with mplayer but are not watchable with the built in player.

Another boost in performance would be to flash your tablet to the latest available OS if you are still running the os that came with the new tablet (that would be os2007 and you want to update to os2008 - again searching does wonders )

Hope this helps.