Well, like the someone said on Discord.... some would argue the point that there's clearly been a drop in performance around the time the PTS goes live just multiple times this year already. Coincidental? Maybe, but it's definitely been questionable. As to why it could be happening, I'm not an expert in the area so I wouldn't even try to say.
But Cyrodiil lag specifically is nothing out of the norm, and considering I'm experiencing lag even in dungeon groups lately.... I'm sure Cyrodiil is an extra mess even when running through it with just a handful of players.
PTS is a completely separate server, it doesn't affect your gameplay experience on PC-NA or PC-EU.
thats not entirely accurate i suspect there will be some common components somewhere along the line.
for example the PTS and LIVE (and staging environments) will share a lot of the common underlaying infrastructure such as the hosts, storage SANS, switches, physical network, firewalls, datacentres - they wont have an entirely different datacentre for PTS - itill be the same kit, different part of their network