Impossible to play H.264 HD video on Avidemux 2.5.6, sound and picture are stuttering badly. Actual editing works though. Lenovo W500 with Intel Core 2 Duo T9400 CPU, 4GB of RAM, Win XP SP3. Any performance improvements coming on the 2.6 or later versions?
H.264 try 2.6 experimental found: http://avidemux.razorbyte.com.au/ or here http://avidemux.org/nightly/
The mantra is don't expect Avidemux to be a video player. That will never be what it is designed to do; however, you can get better performance. If you go into your preferences -> video (i think) there should be a drop down list where you choose OpenGL and below that an OpenGL checkbox. Set them both, hit OK, then restart avidemux (not strictly necessary but it always works).
Thanks, switching to OpenGL cured the problem (almost)