Sounds to me like it could be something in your project is single-threaded (using 1 core of CPU, not all cores simultaneously). Magic Bullet filters and possibly certain Adobe effects too.
Tell us more about the details of this project?
Also, have you set up MPE properly for the GTX 285?
Paul, without details it sounds like you may have a disk bottleneck. What is your disk setup? As Harm said run our PPBM5 benchmark. What version is your Premiere? How much memory?
In a personal benchmark that I am using (AVCHD to MPEG2-DVD with lots of heavy GPU accelerated effects and options) my GTX 285 scores 30 seconds encode time and my GTX 680 scores 18 seconds. With this same test but using CPU only it takes 260 seconds to encode and this is with a well overclocked hex core CPU
Yes, i thought it was multiprocessor set up in AE because there are some dynamic linked comps and it's a fresh install of win and CS6 and i hadn't checked the multicpu box. Having said that it didn't make any difference. Something in there is locking it so it's a case of turning things off until i find it. There are MB Looks but i've not had any issues with CS5 and that. I thought it distributed frames to cores so that it was all single threaded on one core?
The disks are okay, my scratch is a simple RAID0 but never had issues before
The PPBMS benchmark is a good idea though, i can compare with that too
thanks, will keep you posted.