How resource intensive is running iperf3 in server mode

Are you saying you want to have iperf continuously running in the background?

If so, I would advise against it. Iperf measures network bandwidth by pushing as much traffic through as possible. All of that traffic constantly using the bandwidth will make everything else on the network slow.

There are 2 options that would be better plans:

1. Synthetic testing. This is basically anything that simulates things you care about on the network, run periodically. For example, you could test login times to NextCloud every 5 minutes, or you could test DNS resolution times. Two enterprise products that do this sort of thing are ThousandEyes and Kentik. I’m not sure what FOSS tools can do similar things. Iperf could be a part of that if you really want, but I would only run that once at 3am.

2. Instrumenting what your systems are actually doing. This starts with monitoring tools. There are tools like Nagios that can measure how all of your systems are performing and displays it in graphs. You can dig deeper with other instrumentation tools (id recommend looking up Brenden Gregg for examples of what’s out there) that you can then combine with your basic monitoring for even more insight.

Please do your own research on tooling. I do this professionally in the enterprise space, but I’m not up to date on the available FOSS tools.

But bottom line, don’t kill your bandwidth trying to measure your bandwidth

I use this at home :

https://hub.docker.com/r/linuxserver/librespeed

It's basically an iperf with a web interface, pretty handy when I want to test something from my tablet or smartphone.