What you've shown in the video looks like a stall or some error, not a speed issue.It looks like the data isn't actually sent at all.

As for web speedtests - our networking stack is unfortunately not modern enough to give you better speeds (although one can workaround that by proxying traffic through a local linux machine, see Wayfarer's FAQ about that). This does not affect local traffic, though. That one will be CPU-bound, but would still be way faster than traffic to a remote server. And if your NAS enforces SMB3, your speed will also be affected by en/decryption of all data being sent.

I'd need a packet capture to be able to say more.