There are no reasons except that my computer feels slow. I used to change my mac pros and power macs every 3-4 years, and now I've had this one for close to five. It works fine, just getting slow.
Slow in what exactly? What do you use your MP for?
Post how much RAM do you have and OS version you're running.
I have W3570 currently in my 4,1 and 16GB 1066 MHz (single channel) RAM + overclocked AMD 7870XT 2GB.
If you'd me want to do some benchmarks/tests, PM me or I can post results here if you prefer.