Descend 20% faster without going faster!

line choice is not what I would put as the biggest contributor to this.

there have been a few places with descents short enough that I could essentially session the whole thing. what made a given run faster? riding it smoother. less variation between max speed and min speed. that typically meant my top speeds were lower, but it also meant my min speeds were faster.

If I tried to go balls-out and just "go fast" that typically meant my average speeds would be slower because even though my top speeds would be faster, there'd be spots where I'd brake more/harder and wind up going slower overall. and since speeding up takes more time and effort on top of it, it brings the average down. sometimes line choice is involved here, but at least as important, if not moreso, is how you're riding a given line. are you dipping into your toolbox of skills optimally? are there skills you're missing, or are weak on, that hold you back?

what did I learn from all that? not much, really. sometimes it's fun to go balls-out on a section. but sometimes it's fun to dial it back and do a super clean, smooth run. I also ride for fun. so I don't really give anything more than passing interest to my downhill speeds.

I don't think it's all line choice, although obviously that can make massive differences. There is a lot to be said for familiarity. When you get stronger and lift heavier weights in the gym, it's not soley an increase in muscle mass or your laser focus that does it. Strength gains can be from nervous system adaptation where your body learns through practice how to be more efficient.

If it's downhill and your fitness isn't as much of the engine as gravity and terrain are, I'd agree with other posters that your repetition has given you some gains from knowing when to brake, how long to brake, and what speeds to carry into sections.

It's how the same old trails we've ridden can keep seeing PRs. At a certain point each of us have reached or will reach a plateau with our fitness, but it's that constant grind of improving efficiency from repetition that lets us shave a second here or there. It probably isn't even conscious. We've ridden a trail so many times that our bodies just adapt to all of the physical parts and puts itself on auto-pilot for most of what was sensory overload as we paid attention to every detail and spent time making decisions on the first laps we ever took.

That familiarity allows our conscious mind to process other things that might not have had enough room to be processed if it weren't for familiarity, and we can make better choices on things that might make even more improvements.