Default [Q] What makes the tablet slow?

After I do a fresh install of cromi on my tablet, it's just excellent, fluid and smooth. After a while, the performance seems to detoriate. It has done so for about every version I tried and the same goes for my phone (An i9300 running slimkat). So my question is this: What makes an android device inherently slower? Is there a way to see what apps may slow down the performance?