Multi monitor issues are purely on your distro - and are pretty easy to fix. At least for me on arch and bspwm (I haven't touched a Debian based install or full DE in years), setup was as easy as making my randr script run when my WM starts up, I imagine it's even easier with a full DE.

For 2.5 gb/s internet... I've never run into any problems or even had to configure anything. Fresh barebones arch install with lan, 2.5 gb/s out of the box. If you're getting less (my guess is 1 gb/s?) it's almost certainly a hardware issue (motherboard/network card is only 1 gb/s, port on router and/or switch is 1 gb/s, etc)

If you're having trouble with something, I highly recommend searching for the problem after checking a relevant wiki (archwiki is an awesome resource if you're on arch). If you're having issues you can't find problems to, feel free to shoot me a message and I'll try to help you out. I'm no expert, but I've been exclusively on Linux for 3 years (since I graduated and no longer was required to be on windows at all) and haven't run into any issues that I didn't find a relatively easy fix for)