Slow Video Playback

Hi All
This is my first post on ZT so apologies if I’ve missed your usual protocols etc.
I run a photo library on my NAS (running on Linux Mint 22.1) at home and there are no issues watching video clips when I’m at home.
However, my in-laws (who I’ve setup to connect to the photo library via ZT) struggle to watch the same clips: it constantly pauses to buffer. We both have very good broadband speeds (download 50-70Mbs, upload 20-25Mbs), so it seems that Zerotier may be the bottleneck - is there any setting I can change in Zerotier to improve this issue?
I’ve already setup a managed route to point it to my broadband/WiFi router but that has not helped.

Many thanks