Q: How can I increase the performance of my Mac

Yes, of course you can.

The only problem is you are not giving us enough information to help you.

How much RAM does your iMac have?

How much free space is there on your system disk?

What applications do you use?

When using which applications do you notice the drop in performance or is it all the time?

Allan

See:

Mac Maintenance Quick Assist,
Mac OS X speed FAQ,
Speeding up Macs,
How to Speed up Macs,
Macintosh OS X Routine Maintenance,
Mac troubleshooting: What to do when your computer is too slow,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X, 
Five Mac maintenance myths and
Myths of required versus not required maintenance for Mac OS X for information.