For device slowing down you can take the following actions-
1. Clear the browser cache.
2. Move the content from the phone memory to the memory card.
3. Take a backup of all the messages using Nokia suite & delete the excess so that you can free up some memory. In future do not let the message count to go beyond 500 so the phone functions remain smooth. Please note that depending on the number of messages the backup & delete may take even hours.
4. Try using a theme that is smaller in size. The larger the theme size the slower the device becomes.
5. If still the problem persists you can reinstall the current firmware using Nokia Suite. But do take a complete back up of the phone or else you will lose all your data. Also if you've set a password on the memory card & do not remember it then do take a back up of card content also as after firmware reinstallation you will not be able to unlock the card without formatting it.