Extremely low fps (3-4) while CPU load low (10%) and no drops on 720p+ content

Hi,

I'm trying to figure out why I'm getting so low FPS (say 3 to 4 FPS max) on my 720p and 1080p content (both h264 and XviD) when playing with XBMC on my second computer. All 480p h264 media and other SD content (DivX / XviD, etc) seem to work fine. Screenshot here: http://i.imgur.com/aJqqLCy.jpg

I feel this is some kind of software or configuration error (quite possible on my side), as the same content plays fine in MPC-HC (installed latest CCCP with LAV decoders) but I just can't figure it out.

Some facts:

• Use fullscreen window rather than true fullscreen is disabled
• The same video works fine in MPC-HC on the same PC, also with DXVA disabled
• I prefer not to enable DXVA because upscaled SD content looks aweful then
• All media is working fine on my main HTPC. Both run a fresh install of XBMC Frodo 12.1 and are configured the same
• The XBMC installs share a MySQL database (hosted on the main HTPC that also serves the content)
  

Some PC specs:
Windows 7 32 bit
Intel® Core™2 Duo E6300 (2M Cache, 1.86 GHz)
3GB RAM

While writing this I decided to copy the XBMC folder to a different place and ran XBMC with the -p attribute and now it plays everything fine. So it has to be something in the configuration of XBMC. Does anyone here have an idea what it could be?

Cheers!