Spoiler warning: Some Celeste mechanics, strategies, collectibles, and scores are described. No plot spoilers.
I've been doing some casual Celeste speedruns and my best run so far is 0:56:54, 94 deaths, which puts me in the top #1000 on speedrun.com. I had the BEST chapters 5 and 6 but the WORST chapter 3 (kept dying on a long Oshiro room) and the WORST 7 (big plate of spaghetti).
I'm not looking up anything online, I'm trying to find and figure out strategies shortcuts myself that are within my skill level. There's a couple of other people in a small room online that are also doing this and I'm sharing my times and strategies with them, and they are sharing back. They told me about a jump in mirror temple that lets you skip getting 2 keys which takes about 2 minutes to do. It's really easy to skip.
After the mirror, in the section where you're supposed to unlock two key blocks and then ride the red bubble to the left through the tentacle tunnel, sit in the gap before the tentacle tunnel, hug the left wall in the gap, then do the following inputs: dash right, then hold left, then once you start moving left, press and hold jump to launch yourself over the tentacles. Before you land on them, dash up-left, and you should barely be able to make it to the left wall, clearing the gap.
I don't know how long I'll stay interested in this for, but it's been a fun time so far.
On my main file in Celeste I have all hearts, 196 strawberries, the Farewell stamp, WOW, 20018 deaths, and 116:29:35 play time. My Farewell records are 82 deaths and 36:12.