The biggest contributing factor is available resources. The higher quality the flash video your watching, the more resources you will need to run it smoothly.
This is were Slitaz wins over most of it's competitors because it uses very little resources compared to Ubuntu or Arch even if you use LXDE with both. So running Slitaz means you have more RAM, CPU and GPU time available for your flash videos. This equals better performance than a (slightly) more resource hungry OS like Arch or Ubuntu.
Yeah, you'd be surprised. I used a game client which I converted over from the Ubuntu package. Same libraries and files and everything on the same PC. This client runs a couple of frames faster on SliTaz than on Ubuntu. The only major difference is that SliTaz uses 10-20% less resources RAM and CPU time than Ubuntu did. So it shows that every little bit helps ;)