Are you trying to serve higher than 1080p? I have heard that 4k can be rough, but I avoid that personally because it also dramatically limits the amount of media I can store. Not sure of your actual bandwidth, but the raw bandwidth required on regular 1080p streams should be relatively light while utilizing modern codecs and you can also limit the per-stream bandwidth in your remote access settings.

I’m in the US, but I have pretty shit upstream bandwidth and I’ve served 7 people at once before. It pushed it a bit, but plex managed it. If you are doing 4k content though, yeah, that’s going to be rough with anything and I have no good advice.