My blackberry has become very slow all of a sudden.

Applications on the blackberry do not close when opened, each application you open consumes memory. Also the working memory and the memory to store programs are shared unless you have installed a microSD card. Remove and reinsert your battery, if a hard reset speeds the phone up your problem is open applications. If it does not, your problem is that you are out of space and need to remove some applications or install a larger capacity SD card.