I want to share some excitement but I can't really explain it

I have a 2.9 year old Dell XPS 1645 which broke and under special warranty got a the thing rebuilt. I removed the SSD I was using, and refitted the SSD upon receiving the rebuilt tank.

Win 7 64 bit home premium, 1st gen i7, 6gb RAM 1000~ Mhz, Radeon HD5730

Before rebuild:
- takes around 90 sec to boot
- if I hit "Back" when viewing pictures on MS picture viewer (the default program when you double click on a jpg) it takes 2-5 seconds to show the previous picture, going forwards is ok
- there is a few seconds lag between double clicking an app before anything starts to happen
- there is a lag everywhere in everything

After rebuild
- takes around 25 sec to boot
- hitting back on MS picture viewer gives me instant pic
- no lag noticable

Seriously? Why was my hardware causing slowness when old?
Sorry I didn't do any benchmarking etc but the clockspeeds are the same, turboboost working (I think - no reason not to) etc  ...