You should try to find out why it is running slow in the first place. Format should be a last resort.

Go to Applications/Utilities/Application Monitor. Are there any unknown apps that is taking too much resources?

How about free storage? If it's almost full, it can significantly slow down your system.

In any case, before you restore anything, make a full backup (e.g. Time Machine) or clone your hard drive (e.g. SuperDuper / Carbon Copy Cloner) first.