We are trying to progress vSS HMs and we have had to cancel all of our raids. Some of them lasted a little longer before things became completely unplayable, but we are now at a point where we cannot get even one try of Lokke in before giving up. Here's a list from the top of my head of things that tell you your instance is already botched (from Lokke because admittedly Yolna is easier to do while the game is buggy so I don't remember it as much):
Tombs get telegraphed but don't spawn, one of them does while the other doesn't, or they 'spawn invisibly' and then suddenly appear a second before shattering.
AOEs from Lokke beam are spotty (sometimes some people don't get it, sometimes none of us get it). Lokke starts bobbing his head up and down and the laser clips through the floor as it follows his mouth.
Lokke stays in the air indefinitely, either doing extra rounds of his big circles or just hovering as he does closely before landing, generally lengthening phases.
Lokke skips mechanics such as the lightning-path aoe, shortening time to kill atros before beam.
Frost atros dance around in one spot, turning around wildly, seemingly switching between following tank and de-engaging. If you've ever been near mobs right before the server goes poof, you know what kind of dancing I'm talking about.
Our entire group had a 20 second DC where connection to server was clearly severed (everything is stuck in idle animation, but nothing and no one can use skills or move around, char model may go invisible), although those of us that didn't log out in that time did eventually come back.

And most importantly, skills are absolutely not firing. Not just a bit, not just some skills. You can watch your action bar and see that it registers a pressed button and nothing goes out. Tanks are struggling to taunt cause it just won't take. Sometimes this is just a massive delay -- ie suddenly after 10 seconds my char then spams 5 healing springs in a row as if it remembered the earlier input. Sometimes it never comes. AOEs are also suffering their typical position desync. Ping spikes are way too common and even outside of a spike, ping is generally high.

This is just Sunspire. I've found it to be the worst offender right now, but other trials aren't doing much better. We usually try to do something simpler if we can't do vSS at all, but even when trials are doable it's just not fun when you can't do anything half the time. Anything that requires bashing is also really borked right now.

I've been through a lot of crap with ESO's performance and usually try to take it in stride. Yeah it sucks, but usually it's more specific and it ends up being temporary. With this patch, I wanted to wait and see if things mellow out, but these issues don't really strike me as related to the performance update, as that was based on loading assets and patching... And I don't see people being as vocal about this as I would have expected. Maybe this is because it is much worse in trials, and that's not as commonly run as dungeons. But it's hard for me to imagine that this could be just us, because at this point it's been weeks and it's always the same. It's stopped being a fluke. We honestly try to make things work no matter what, we're used to some level of *** performance, so it reaching the point where we have to cancel our raid (with 'cancelling' I do include attempting to run something else instead) is kind of a big deal. We really don't want to be doing something else. It's just getting tiring. And I'm starting to get worried that there's no fix in sight. This hasn't just been one or two raids, this has been every single time. Sure not all the examples I listed happen in every raid -- it's more like, every time we try we find a new way the instance can be bugged. It's just not okay that vSS becomes legitimately unplayable for us in less than 15 minutes of being in the instance.