so basically i wanna speedrun BTD6, cause thats the only thing i havent done in the game, but my device is slow, so i lose seconds between rounds, what should i do?

Im not an expert but I think the best way is to probably decrease the number of apps running in the background and decrease the resolution in the game (this helps a little). I also think that you might not need a timer, since you would count the frames of the speedrun anyway later (not sure about this...need to wait for better answers, maybe from one of the mods maybe)... so if that is the case, that would probably help a little as well.

Other than that, probably get a better device I suppose.