Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
There are mainly two reasons for this problem. Either the system files are corrupted/glitched or the hard drive itself is failing. Failing hard driving leads to constant delay in reading and writing from and to the hard drive and this will affects the performance of your Mac. So first I'd recommend to backup all your important files to an external hard drive.
Then we can do some troubleshooting steps to identify and solve the issue.
First reset the PRAM and NVRAM of your Mac. Follow the steps below to reset the PRAM and NVRAM
Shut down your Mac
Restart and immediately hold down Command-Option-P-R keys at the same time
Hold the keys down until the computer restarts and you hear the startup sound for two or three times.
Release the keys and let the Mac bootup.
Then run Disk Utility from Single User Mode. Turn your Mac off. Turn it back on and immediately hold down the Command (Apple) key and the S keys at the same time. Then type the following command when it boot into black screen with white letters...
(note the space between fsck and -fy)
If you can't run /sbin/fsck -fy then type only fsk -fy
Hit return and the disk utility should run and let it finish. When it finishes type reboot and hit return. Does that improve the speed of your Mac ?
Please let me know if that didn't work, so we can continue troubleshooting.
Remember, this is not my final answer. If my solutions didn't work please don't rate me bad or poor. Instead please reply back to me and I will continue to help you until we fix the problem or you are satisfied.