In my experience most problems with linux are at the intermediate level, i.e. things like setting up university/work vpn, installing games (with wine), getting used to different applications for office stuff.
This is all stuff that many people have to do that can be hard to achieve if you only have guides for windows/mac
Im basically exclusively using Firefox on my phone and loading websites is pretty fast, the app just feels less responsive when scrolling compared to chrome/bromite
That really depends on what kind of computer you are using and how fast your internet connection is. Also a desktop computer should be (for most people) as little maintanance work as possible and having long update/install times really stands in the way of that.
Honestly I tried many lemmy apps and most apps are worse than just the mobile website, except for sync which is just slightly better