I've notice that these smart phones work just like mini computers. They might not get the virus like a PC does, but some application(s) tend to cause corruption that slowdown the device performance, thus causing noticable lags. Occasional powering off the device refreshes the system. You could also uninstall unnecessary app(s) as well.
Haven't really had any slow downs lately. probably you have a lot of apps running the background that multitask all day with notifications on and stuff.
With the open platform it seems like just about anyone can write an app and upload it to the market. While that makes tons of good apps available, that also makes for some garbage ones. I never download more than 1 app at a time and make sure I test it out to see that it doesn't cause problems.
Thanks,
Im gonna try to go through and delete my apps I dont use. Ive already restarted my phone 3x's today cus it was lagging so bad I couldnt even text. Hopefully this helps.