Just the other day I replied to a Reddit thread, and told a gamer who experienced periodic frame drops in CS:GO to check the other system processes. He protested that he had nothing else running, but elsewhere in the thread a Task Manager screenshot revealed that Windows 10 Smartscreen Filter was chewing through files and bottlenecking the HDD(a problem I've also experienced).

Basically, desktop Windows has crossed the threshold of "needs SSD" to perform adequately. And in theory that shouldn't impact the performance of an older game, but it does, because what happens next is that the scheduler misallocates process time and throws off everything.