I’ve been having the same issues, and tried this workaround for a different issue:
https://forum-en.gw2archive.eu/forum/support/tech/error-42-0-9001-3927/10994
Seems to have fixed the problem for me, I just did a 6 hour session with zero lag.
My guess is it’s the ISP throttling port 6112 in peak times, I think it falls within the range of ‘file sharing’ ports. They will never throttle port 80.