Please don't say to 'close running apps' because I did, but we all know that doesn't mean anything.. (speirs.org/blog/2012/1/2/misconceptions-about-ios-...)
On my normal configuration I have about 350 apps installed
I'm really not looking for a fix for my current configuration, I'll just do a factory reset. All I want is an explanation..
UPDATE: I found the main problem: It all has to do with IOS's caching of app icons in the homescreen. From Epic's Mark Rein on Infinity Blade 2's crashing on iDevices with insufficient memory.
"For example if you have a large number of apps installed iOS must cache all those icons to give you smooth performance when you hit the home button. Unfortunately the machines we tested on didn't have massive numbers of apps installed on them so we think this how we think we missed this particular condition." kotaku.com/5864309/the-most-popular-game-on-itunes...
So the number of installed apps do indirectly affect performance.

