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This usually happens if the system files are corrupted or if there is any issue with the hard drive. It could be the hard drive that is failing that lead to slow read/write speed. So first, I'd recommend to backup all your important files to an external hard drive before doing any troubleshooting.
We can run Disk Utility and see if it works or not. Turn off your Mac by holding down Power button for 10seconds. Then turn it back on and immediately hold down Command and R keys atsame time for 20 seconds and this will boot your Mac to the Mac OS X Utilities screen. From there, click Disk Utility. Select your hard disk at left column(Macintosh HD or hard disk name). Click First Aid button and run it. Repair if asked.
Then restart from Apple menu. If that didn't work, then turn it off. Turn it back on and immediately hold down Command, Option, P and R keys at the same time for 5 seconds, then release the keys and immediately hold down Shift key for 20 seconds. Does it boot into desktop ? If so, restart normally.
Please let me know if that didn't work, so we can continue troubleshooting.
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