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.
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.
Connect your phone to a computer & select Nokia Suite mode. If you do not have Nokia Suite on your pc you can download it from here. Now open the Nokia Suite & click on the “green icon of arrow pointing downwards”.
If any updates are available they will be shown in ‘green’. If not you may reinstall the current version. Reinstall option is shown in ‘blue’. If no option to reinstall is shown, please wait for sometime it will become visible. Then the Nokia Suite will download the new software version of the phone. After it you will be prompted to take a complete back up of the device content. Please do not skip this step as reinstallation will erase all the personal data from the phone like contacts, messages, calendar entries etc.
Please note that if you have set a memory card password & you have forgotten its code then take a backup of the card data also as the card will be locked after update and can only be unlocked if you enter the correct code or by ways mentioned in this thread.
Also note that while updating the device ensure the following things-
DO NOT DISCONNECT THE PHONE UNTIL UPDATE IS COMPLETED.
Also check out this video on YouTube with instructions for update.
If problem persists, please visit the nearest authorised Nokia Care Centre.