@WiSo, I'm sure that things are not so bad.
I'll add 'XBMC 64-bit experimental' to log.
About performance. I'm not sure that web browser is most CPU hungry app.
I did my own test on video performance. You can reproduce it.
You need:
* heavy test clip (I used Ducks.Take.Off.2160p.QHD.CRF25.x264)
* GraphEdit (32 and 64 bit) or GraphStudioNext (32 and 64 bit)
* LAV Video Decoder (32 and 64 bit, same versions). You can try FFDShow.
* PC with Windows 64-bit
Render media file in GraphStudioNext (you can just drag-n-drop file).
Disable GPU decoding in Decoder.
Press play and wait until it finished.
Check 'Quality' tab in properties of Video Renderer.
In case of 64 bit decoder, I got 29.94-29.97 FPS and 0 frames lost. (CPU is Core i7 950)
In case of 32 bit decoder, I got 27.31-28.04 FPS and about 4-9% of frames lost. Several test runs.
I try to limit number of threads for decoder to 4 threads. Then I got 19-20 fps vs 17-18 fps
And unexpected result: with enabled GPU decoding (HD 7850) I have 29.95-29.67 and 0 lost on 64-bit vs 27.0-27.6 and about 10-12% lost on 32-bit.
Edit: looks like GPU decoding is just not working, ether on this clip or in GrapStudio.