Have you tried running the free version of malwarebytes just to make sure the computer is clear of any spyware problems.
Defrag maybe.Or drivers needing updated.
Give c Cleaner a run to remove any old crap on the computer.
Check Task manager to make sure there are no programs using up most of the CPU.
if nothing running system idle process should be showing about 96% and that it normal.
If anti-virus is running a scan at start up change it to a time when you want it to run.