Thanks for the tips everyone.
I ran the latency checker, and yes, a completely red graph resulted (I am streaming on this PC for the moment).
I disabled the Avast firewall, and the problem seems to have disappeared.
My PC was almost unusable, I had a rought time just trying to open the Avast settings to disable the firewall.
I don’t know if this is normal for a protection software, but this is definitely not acceptable.

Thanks again for pointing me to the cause of the problem.
Think I’ll start by permanently disabling the firewall.