SWAP partitions are used to support virtual memory. Actually, data takes to swap partition when there is not enough RAM to store it properly specially when system is under process. Generally, it is equal 2x physical RAM for up to 2 GB of physical RAM (specially on Linux distro), but never less than 32 MB.
Sometimes over utilization makes the system slow and I myself experienced the same problem. I pass the command like: swapoff -a (for over utilization or overcome the slowness) if u make it again pass as: swapon -a
For more details see man swapoff / swapon
If u want to see per processor swap utilization then type as :
ps -ef