I think that u've so many data saved in ur MMC. Like images, sound, etc that will make phone runs slowly.
-Try to delete/move them to PC.
-Try to delete inbox, log (also reduce log settings 2b 10 day or 1 day).
-Upgrade the firmware. Go to nearest Nokia.
-Use some 3rd app such as TaskSpy or FExplorer to compress ur memory while running.
-Don't forget to always use Exit to an app so it not running again in the background.