Maybe run this and see what download speeds you are getting.
Try command prompt and type in ipconfig /flushdns It should tell you that the DNS Cache has been flushed if so reboot and see if it makes any difference.
If not running Adblock Plus with Firefox maybe give it a try.
Go into Taskmanager .processes and under CPU find out what is using up most of the CPU.
If nothing running system idle process should be showing about 97% and that is normal.