I do come back and revisit old pieces. I actually play parts of the Clementi Sonatina just to warm up sometimes. I just pulled out a Diabelli piece I hadn't touched in years and can't believe how much more comfortable it is to play now. It feels like I can digest more while reading it. I think now after 5 years things are a little easier. It's hard to believe my first post here was almost 5 years ago now. Seems like yesterday!
You have to be careful, though. Learning a piece that you already had significantly worked on before is much easier the second time and you may even recall parts of it with a minimal read through. This is possible even after years of not playing a piece. You should try to learn a new piece of similar difficulty to really assess how much you progressed. If you practice diligently you will be pleasantly surprised how easy it is to learn a piece that is below your current level.
This past year I tried to focus on more relaxed playing and by doing so I think that has allowed me to play faster. I don't try to incrementally tick up the tempo anymore really. I just play at a tempo that fells the most relaxed. Perhaps this is the difference.
I was never really concerned about tempo except to force myself to play slower while practicing in order to avoid making mistakes. I play at a tempo that feels comfortable and allows me to play without fumbling all the time.
This depends on two different things. First, there's the inherent maximum speed you are able to achieve, which is the limit of your technical abilities. If you can't play scales at 120 then you simply can't play a sonata that has long scale runs at 120. This limit increases over time but very, very slowly. It took me about 6 years to play scales at a tempo that you could reasonably call "Allegro". Every few months I make a little bit of progress but it's almost unnoticeable in everyday practice.
Second, is your ability to be mentally ahead of what you want to play at a given tempo. Most of the time when you stumble it's because of a mental gap that has you unprepared for what comes next not because your fingers are not able to play that fast. You can easily prove it by playing the most difficult 1-2 measure passages at maximum speed and comparing that to the speed at which you're able to play the whole piece.