Just reinstalled OS X and it's so fast I can't believe it. Hope it stays like this after I've reinstalled all my apps!
What imac do you have?
A new machine always runs faster than it's predecessor! Over time and familiarity, it appears to slow down. However, once the used space on the HDD gets up to c.80% of total capacity, the machine will start to lag a bit. That's the time to have a spring clean and/or move unnecessary data onto an external HD.
Macs don't slow down like Windows machines that suffer from the ever-enlarging Registry file!