iPhone 4 animation/transition lag

[Wikiboo]

When I unlock my iPhone or sometimes when I open or close apps, animation sometimes lags.

I've started noticing it more and more and I'm a little worried.
I know it's not a big deal, and it's just a visual animation of opening and closing apps, but I wonder if it's happening to you guys to, and why does it happen?

 

[miles01110]

How many apps show up in the multitasking menu? Sometimes things slow down a bit and I find closing some of those helps.

 

[Wikiboo]

I've shut them all down, but sometimes still animations lags, not all the time

 

[gloss]

I've shut them all down, but sometimes still animations lags, not all the time

I'd been experiencing some major lag/slowdown with my iPhone 4 recently, and someone suggested that I do a force reboot by holding down the sleep and home keys for about 15-20 seconds, until the device goes black and the Apple logo comes up again. Half a dozen regular reboots and even a restore didn't help, but this did the trick for me. Works like new.