Sorry bud, but you're looking for a different answer than the correct answer because the correct answer doesn't fit with your conclusion.
Windows 10 hasn't gotten slower. I've had the last 3 major updates on the same hardware with no degradation in performance.
There is something wrong with your setup. Either a software (my guess) or hardware issue. And, like it or not, mechanical hard drives are slow as ****. If they're 5400RPM drives, they're literally bottom of the barrel and were slow the day they were made. Don't take offense to it, it's just a fact.