If the slowness is due to misbehaving software, this article has some first steps to try. Creating a new user account and seeing how it performs is a good one overall.
https://support.apple.com/en-us/HT201516 But before you do that, make sure you have a complete backup of everything important. The last two macs in my house that "got slow" eventually ended up with dead hard drives -- and Apple's diagnostic hardware was useless as tits on a bull for... well... diagnosing the problem prior to their death.
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