I had a Captivate for 3 days before returning it for my GS2. Mine wasn't laggy but instead the internet was just not connecting 90% of the time. The only time my GS2 gets laggy is when my available RAM falls below 300. But apparently I have quite a few memory hogs running as I only get about 560 of my 1024 from boot. Mine eats through the battery too. I average about 5.5hrs bat life.
I heard that the GS3 will have 2gb of ram instead of 1. I dont feel 1 is sufficient. Anyways, whether your GS2 will be laggy depends on how many ram intensive apps you run. I'm using Go Launcher EX with some fancy effects too. And I like my live wallpapers too much.
Right now my ram is sitting at 320 & my keyboard is starting to get quarky. Ill prob have to reboot in a few hours.
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Sent from my Samsung Galaxy S II