My iMac had been running slow and freezing

So I have and iMac, it was running slow and constantly freezing. I checked the space left on the memory and it was like half used and the RAM memory was also ok. So I took out some file to an external memory and erased everything and reinstalled the system. That also didn’t work because it wouldn’t load when it restarted. So I erased everything again and installed maverick since it’s what the computer came with. It worked and I could get in. But it is still slow, the apps take long periods of time to respond or don’t respond and it’s in general slow but a bit faster than before. Help please.