Hello Binkabi. When devices slow down a bit, it is for one or more of several reasons.
Resource drain can come about from background services and syncing running while you're trying to play a game or use your web browser, etc.
A good thing to start off with is a bit of analysis of what's running in your device. Go in to the application manager in menu > settings > applications, and have a look at what's running in the background. You can stop some of those services, but it's also good to configure syncing apps and widgets to sync on demand or manually rather than auto sync. Weather apps and widgets and social networking all take up a lot of resources and can slow things down over time.
Some games are just slow on some devices. You may want to have a look at the hardware acceleration settings to see if there are options for faster or slower rendering (I don't have a GS2, so I can't give you the menu for that if it's there).
Often a simple reboot of the device clears things a bit. But first it can help to go into each app and widget (again, in the application manager) and clear caches on them all.