I noticed my Google Nexus S phone running really slowly, and initially blamed the upgrade to Anrdoid 4.x running on a two-year-old phone. The same thing happened to the ASUS Transformer TF101 tablet; it too got an upgrade to Android 4.x. It too began running slowly.
I puzzled over the problem, trying different attacks on the problem. Was it certain apps I had recently installed? (I uninstalled them.) Was it widgets taking up too much CPU time? (I shut off all widgets.) Was it the new "butter" animations introduced by Android 4.1? (I set them to 0.)
Nothing helped. The slow app response, the frequent crashes continued.
Then I remembered something: the ASUS tablet ran just fine after the update to Android 4.x. There was, however, one significant change after the upgrade: I installed the Go Launcher HD software, which replaces the stock launcher. I really liked the interface of Go, but wondered about it...
After I uninstalled it from my phone and tablet, both returned to their normal speedy selves. I guess Go just chews up too many CPU cycles.
So if your Android runs slow, blame the launcher.