I finally had some time to do a few network traffic experiments to assess the impact of traffic on sound quality. Here's what I heard when I started loading up my very simple network.
First a brief description of my home network:
Slow "high-speed" internet via DSL (12mbps down/1.2mbps up) via a ActionTec 1900C combination modem/router (powered by a JS-2 linear power supply).
From one of the RJ-45 ports on the router part of the DSL modem the path is via UTP copper jumper > OpticalModule (with a Sonore Signature linear power supply further regulated by a two-stage DXP regulator) > Finisar SFP > fiber optic cable > Finisar SFP intalled in my Extreme's SFP cage.
From another RJ-45 power on the modem's router the path is via UTP copper jumper > Startech FMC (regulated linear power supply wall wart) > PlanetTech SFP > short fiber optic jumper > Planet Tech SFP > Startech FMC (regulated linear power supply wall wart) > UTP copper ethernet jumper > Netgear GS105 unmanaged switch (regulated linear power supply wall wart) > the rest of my home LAN, including three NAS, upstairs and downstairs AV systems all connected by UTP copper ethernet cable.
I tried the following:
1. listening to music while running a NAS to NAS back up.
2. listening to music while streaming a Netflix movie.
3. listening to music while running an Extreme to NAS back up.
1. and 2. both exhibited a slight degradation of sound quality.
The magnitude sounded about the same doing the NAS to NAS back up whether listening to local content or streaming tracks for 1.
For experiment 2., the sound quality sounded slightly worse for streamed content vs local files. Not surprisingly given my very slow internet speeds, streamed content took much longer to buffer and begin playing, so perhaps my impatience somewhat colored my perceptions.
3. Running a backup of the Exteme music files to a NAS. This exhibited a greater decrease in sound quality than either 1. or 2. Music was still far from being unlistenable, however it was still noticeably worse than 1. or 2. I'd say that the sound quality hit was on the order of switching from TAS to Roon -- grayer and flatter than TAS playback when not doing network intensive things.
So, this is my particular experience with a simple home LAN. Hopefully there is something in this that is helpful.
Steve Z