This Mac Pro 16 GB of RAM, Terabyte of Memory, and 2 x 2.66 GHz Processor and it's running Premiere super slow?
We're all a bit confused here.
I mean, the thing should be running at tremendous speed, and in Photoshop and Illustrator, even Final Cut, it does. But Premiere is slowly running while editing Pro Res green screen footage. I've been having a laggy time scaling PNG files in general.
Any suggestions as to why the speed is not what we think it should be?
I would suggest that you ask this question in the Premiere Pro Forum since only that application seems to be affected. I'm sure there are lots of tweaks in regard to memory usage and where scratch files are kept, etc. that are specific to that program. I use Final Cut Pro X so I have no idea why Premiere Pro would runs slow when Final Cut, Photoshop and Illustrator do not. I'm sure someone in the Premiere Pro forum could help you.
That is definitely not a beast (even in lower case) Mac Pro. That Mac pro sells for $230 on Amazon. A beast Mac Pro sells for over $10K.
If you want to try and make that system work with Premiere and ProRes I'd suggest starting with installing a couple of extra drives, stripe them as RAID-0, get a backup drive for your new RAID, set Premiere to use the RAID for all of your video media.
Also your green screen keying is probably wanting to use your GPU for speed, but your card may not support Mercury GPU accelerated playback.
Before any upgrades, first consider the cost and whether you'd be better off just purchasing a newer computer, or alternatively just stick with FCP 7 or whatever NLE is working for you on that system.