I’m streaming video from an SMB share on a 7-disk RAID Z2 array using VLC. The video pauses intermittently, and I can see these pauses reflected in disk I/O. Streaming multiple videos simultaneously doesn’t seem to worsen the issue. This problem started recently, possibly after a drive replacement and resilver, though that might be a coincidence.
How do I figure out what is causing this?
What OS is the client? Windows? Linux? The other one?
Windows
Do you notice any difference if you set the initial buffer (in VLC) to a high amount, such as 10 seconds (10000 milliseconds)?
VLC → Tools → Preferences → All (bottom corner) → Stream output → Stream output muxer caching (ms)
I tried that and increasing the network cache time. It will smooth it out a bit, but every so often there is still a 5-20ish second hitch. Maybe I will try connecting with another cable and see if it is some network weirdness. I get good numbers when I do iperf though.
You mean BSD? 
Can you try having a ping running in the background at a rate of 1 ping/s while you stream video?
It might help show if it’s a network issue.
Some / all hardware details please. Especially this:
There are multiple gotchas that can make things slower. Not that I would think as noticeable as you point out.
I think I figured it out. At least I have had 20 videos streaming at the same time with no problem for the past 30 minutes.
I believe it was Tailscale. The windows client alway launches and connects on startup. I had set it up a few weeks ago and then turned it off thinking it would only connect when I told it too. It hides in the tray even when connected/running. I was still getting gigabit iperf performance over it.
Sorry for owning myselff and wasting time.
