OP/ED encodes often have much higher bitrates than the program content so they stress your processor more. My last computer was a 3.0 GHz Pentium D with an nVidia 6600 that could not play the OP to
Kure-nai without losing sync or dropping frames. It had no trouble with the program itself. I later upgraded the video card to an nVidia 9500 and used nVidia's drivers to offload h.264 to the graphics card. This solved the problem for me. (I use mplayer on Linux for which nVidia has a "VDPAU" driver, but there's an equivalent "
CUDA" driver for Windows.)