Sounds like your system needs some TLC. Overtime the drive builds up junk & gets fragmented both sap performance, Here's what I would do:
Start off by clearing out the old log and cache files which build up over time. There are a few apps that will work here here's one on the App Store: Disk Cleaner Pro.
The next thing on our list is to create a bootable external disk (HD or USB thumb drive) then boot up under it so you can run Disk Utility from it to repair the internal drives permissions & disk. Do make a backup before you do this just incase!
Then next thing here is to free up 1/4 to 1/3 of space on your drive. once done you'll need to defragment the drive. I use Drive Genius there are others.
After you defrag the drive you should see a lot more zip!