Withought knowing your cpu type/speed, amount of ram, gpu ram and how much harddrive space you have left makes it a little difficult to troubleshoot.
Quite a wide range of things can be interfering. Could be hardware/software related.
Also all the programs which run on startup can make an impact, those ones you find next to the time, bottom right corner of your screen. You can right click the ones you don't really need running at the time and close them down. They won't be gone the next time you boot up they'll reactivate themselves again