flatpak is basically linux’s app store. flatpak is easier to install because it’s just a button press and everything on flatpak is safe, and it handles updates for you, so try to use it as much as you can.

any old laptop with at least a third gen intel or something comparable works. you do want to get some extra storage via a cheap old hdd

all intel drivers should be in the kernel, do you have the iwlwifi package installed?

pretty werd because dual booting shouldn’t influence download speeds

run lspci and see which wireless adapter you have

try using any other device on the same network. could also be a network issue if windows and linux are having issues. run a network speed test on your phone and laptop.

what network card are you using? are you downloading from the same websites that you get faster download speeds from on windows?

android is very sandboxed, so anything you do in termux/userland can’t affect the android on your phone (unless you’re rooted)