Right now old laptop as a plex server to my TVs each with a Roku.
What other stuff are you using the network for?
If you've got gigabit ethernet everywhere, and a fully switched network. traffic between host A and B (say, your partners' computer and the NAS) won't interfere with traffice between hosts C and D (the plex server and the Roku.) So there's really nothing to worry about.
If most of your traffic is internet, capped at, say, 50Mbps, then the bottleneck is there - no single node on the network will be pulling down enough data to interfere with plex traffic either, even over wifi.
If all your clients are wifi, then just create a traffic jam on your WAP by copying a bunch of files from a wifi client to your NAS (or something) at the same time you're trying to stream. (WiFi is shared medium, like the old hubs.)