[vdr] Vdr-1.6.0 and mplayer 1.0rc2-4.3.2 : cpu usage ?

[kafifi]

Hello,

I am trying to view HD movies with vdr-1.6.0 and mplayer 1.0rc2-4.3.2.
Of course, because I am using Nexus output, vdr has to resample the files 
to SD (720x576). Even if my vdrbox has a E4500 Core2Duo and 2GB RAM,
1080p movies are jerky (sound and image). After more tests, I noticed
a different behaviour, according I run mplayer alone, or from vdr :

mplayer alone
- the two cores were used at the same level. 

mplayer via vdr
- one core is at 100% and the second is about 40%

Is there any way to tune vdr to run mplayer using hte 2 cores ?
Did I missed something ?

Thanks