1. What things are considering "cheating" in rules?
2. Why is Optifine banned in 1.16?
3. Can you use 1.17 pre-releases in 1.16 speedruns?
4. What strategies shed a lot of time, but are easy to do?

I can answer quite a bit for question 1 from an overview of speedrunning

splicing which is using parts of several runs and putting parts together to pass it off as a single run
Using hacks or using other stuff to give an unfair advantage over others or whatever
A mincraft specific one is using a seed you found before and using that to try and pass it off as a non set seed run

Other people can say some more specifics in this and the other questions as i am not entirely sure on the other stuff i k ow

1. Using set seeds (if random seed), splicing runs, going into creative, using commands like /give, /tp, etc.
2. I think Optifine messes with mob spawning and zooming isn't allowed. Just use sodium, lithium & phosphor as an alternative for performance enhancement.
3. You can't use snapshots or pre-releases for speedruns, since some have weird bugs that can be exploited.
4. Hypermodern is an easy (sort of) strategy that involves going into oceans to loot shipwrecks, treasure, monuments for iron and gold. Bastions are also used to obtain almost guaranteed pearls, obsidian for blind travel, strings for beds, and fire res for an easier blaze fight. I recommend watching Couriway's video titled "20 MINECRAFT SPEEDRUNNING TIPS FOR ANY PLAYER!".

I can confirm that Optifine messes with mob spawning like Gelo said, I did some tests and the difference of spawning changed a lot.