What methods and techniques have you found to notably reduce your completion times in speedrunning, and how do these differ among various games?

It sounds odd but actually practicing choke points and specific strats in runs rather than doing a lot of runs is absolutely crucial. Even in games where I feel like I know exactly what I'm doing and what to do, practicing exploits and setups a lot makes it almost subconscious in runs.

Usually if I want to get a decent time on the fullgame boards, I'll get a feel for the levels first, or I'll just practice the entire game or marathon until I get world record or a good spot.

i lower my times by doing more runs

I play wide variety so

Platform / action game - these mostly just need grinding until muscle emory is better.

RPG - the ones I played, I was first runner so I made a flowchart of minimum needed to beat game > compared to ganefaqs etc > began poking around my save files looking at things that arent really required even though the plot claims they are > only at the end do i work on combat and specifically lowest level possible combat

RTS/4X - these I had to forget almost everything I knew casually and train myself to be ready for autocombat manipulation. Whenever possible base building is skipped if starting army can work.