Alright, I did a second run, 9:56! (I don't know if you have to talk to all the cubes, I ended up doing it)
I also used a different browser this time, and I can confirm that that was my main problem: all the lag was gone! (The double jump timing also seemed easier, but that also might have just come with practice).
This was my platformer: https://noahmakesjunk.itch.io/a-little-time-to-cool-off-post-jam-edition. I hope you enjoy it, it ended up being quite a bit more difficult than I intended while making it lol.