The thing I'd look to are the wait statistics. What's causing the system to slow down? That will tell you if you are stuck on I/O or if it's something else like memory or CPU. Also, you can try SQLIOSim or SQLIO to test the disk subsystem on your server. That will tell you what the hardware itself is capable of. If your queries are then similar in scale, you'll know that you're at the limit of the system. If not, you'll know that you have a bottleneck elsewhere, possibly in code, structure, or in another resource.