If I were you, I'd do a fresh install of Windows. It was certainly true of XP that, over time, it collected so much crud that slowed it down that the only solution was to do a clean reinstall of the operating system. That would be pretty much guaranteed to restore your PC's performance to how it was when you built it. Windows 7 is better from that point of view, but not immune to similar issues.
OnePlus 5 review