Tomorrow I wanted to get an external monitor to connect it to my laptop. It currently has 4GB of RAM and is no longer the best, but it's still okay.
Can it be that if I have both screens running at the same time that the laptop has a drastic drop in performance, overheating or something similar?
No i use 3 monitors on my old laptop with linux
There's a minimal drop in performance that you don't notice, so no problem
The graphics card / laptop should be able to do that easily. They're built for that.