Your issue is the age of your hardware. Running 10.10 or above on a machine that only has a Core 2 duo processor and a standard HDD. OS X 10.9 and above was designed to run off SSD drives.
The machine you have is right at the minimum specs to be running that OS hence why its slow. If you downgrade to OS to 10.6 or 10.7 you will find a massive improvement on speed.