Could you describe your setup more? Are your backend and frontend on the same machine (rule out network problems)? HAve you looked at your frontend log for clues? What kind of hard drive are your storing your recordings on - if it is IDE is DMA enabled? might want to check the speed of the hard drive reads:
replace DISKDEVICENAME with whatever disk device you are using.
sudo hdparm -tT /dev/DISKDEVICENAME
Your processor and GPU should be able to playback 1080p without XvMC. What are your playback settings?