Shifting problems for a long time, I have replaced
1. rear derailleur
2. chain
3. shifting cables and housing
4. cassette

I am able to shift up with no problems. Going from small cogs to big cogs. But it's really crappy shifting down (from big cogs to little cogs). Like it hangs up for a second. I just now replaced the cassette and that helped. The other stuff was replaced last year. This is a mtn bike and I don't ride it much. I checked wear on the chain, it's fine. But maybe I replace the chain again? Is somehow the cable hanging up and that is preventing quick clean downshifting? Any ideas on what to do? I believe that yes, I have all the correct parts.