Here is the log until it wasn't watchable for 30s: https://paste.ubuntu.com/p/mDx7FrmRGT/
root@hostname:~# curl http://PATH.mkv > /dev/null
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 34.4G 0 44.7M 0 0 1102k 0 9:06:13 0:00:41 9:05:32 1516k
Current speed sporadically drops to 0 during transfer.
root@hostname:~# dd if=/PATH.mkv of=/dev/null status=progress
961397248 bytes (961 MB, 917 MiB) copied, 33.4164 s, 28.8 MB/s
So it seems there's something going on with caddy