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I'm newbee, but from experience, for windows 10 it's better to use SSD, you will never facing slow performance anymore., at least for me.
Anyway my brother have slowdown issue on his old laptop couple years ago,. sometimes its get bluescreen, and slow startup loading, and even shutdown took some times. He been using that laptop like 24/7 lol. And i heard tick tick tick sound,.. so after change it to SSD all problem gone,.. its seem the old HDD just about to die. I remember he's so happy after using ssd lol, ~almost 3-5 minutes boot time before and took only few sec / less then 1 minute after using SSD.
As others said, the PC slowdown has many causes.
To begin to understand something more about your problem you could open the Task Manager to see if any process is saturating the CPU, RAM or disk access and posting here a screenshot.