Returning to home screen after sending an SMS or making a call takes up to 7 seconds on an average where as it used to be much faster six months back. Its a Aug 2011 model. System hangs often coming alive with Force Close application message. Why?
Because Sammy's software is buggy! If yours is a galaxy S i9000, then there are ICS Roms on XDA forum which I found far smoother and usable on my phone than Samsung's touchwiz Gingerbread.
If you don't want to go through the hassle of rooting, flashing a custom ROM, then I suggest that you backup your data and do a factory reset.
settings=>privacy=>factory data reset
You can try a custom home launcher, which is another term for home screen, instead of TouchWiz, Samsung's default launcher. Go Launcher is a popular one and is very customizable. ADW is another one that has good performance. Doing a search for "launcher" in the Play Store will bring up multiple options.