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A Mac will become slower due to mainly two reasons. First is, corruption in system files or software related issues like hard drive full etc. Second is, when the hard drive is failing. If the hard drive is failing, then the Mac will act very slow, get constant spinning wheels, long load times of apps etc
It is possible that the hard drive is failing. Their advice to change the hard drive to SSD is a very good advice. Even if the hard drive is not failing, the process is an upgrade. I usually recommend this to many of my customers. Upgrading will give you about 5x or more performance than what you're originally getting when you bought it. To get an idea about SSD, see this video --> https://www.youtube.com/watch?v=f_2Axf5XAlk
So to understand what the problem is, please download and run a check with this app on your Mac --> https://etrecheck.com/
Then paste the report here. Let me know
Note : Since you are facing slow issues, I'd recommend to backup all your important files as your hard drive is probably failing.
Note : This is not my final answer. I strive to provide Excellent service or 5 star service. If you feel you've received anything less or if you are not satisfied or if my solutions didn't work, please don't rate me bad or poor or ok as it will badly affect my job here. Instead, please let me know and I will try my best to help you until you are fully satisfied or the issue is resolved.