You have to check that profile those HD videos are using.
Tegra 2 can decode 720p high and 1080p baseline or main (not sure here), the problem is that Honeycomb is not fully optimized yet to use all Tegra 2 acceleration features.
I think that if you re-encode those HD videos to baseline profile, they should run smooth.
There will be an upgrade from Adobe for flash to hardware accelerate 720p videos on these mobile SoC, but it needs Honeycomb 3.1, it's still not available yet.
HTC EVO 4G:
- Gingerbread 2.3.3 (rooted, CM 7.0.3)
- SanDisk microSHDC 16GB
Asus Transformer TF101:
- Dock TF101
- Honeycomb 3.0.1 (stock)
- SanDisk microSDHC 32GB