Like a lot of players in both PVP and PVE, I've been experience multiple occasions where skills simply refuse to fire. It's mainly been ultimates for me (most noticeable in vMA...thanks ZOS).
I recently made a gear change on my stamblade and wanted to test my DPS on a dummy. I hadn't parsed since U25. I went to my own house were I was the only player. My FPS was pinned at 100 and my latency was 55-65. Things just didn't seem right. Weaving wasn't working well, my DPS was low, and things were just clunky.
So I systematically started removing skills. It turns out that my spammable, Surprise Attack was only firing once every 3 presses. Even sitting there hitting that one skill every one second wouldn't work. It would fire once, then the next 2-3 presses, the button would blink, indicating the client received the command but the skill wouldn't go off. I tried a different dummy in a different house and that fixed things for a bit but things quickly degraded again.
The game literally couldn't handle me activating 1 skill once per second. I'm not sure what the "performance upgrade" was supposed to do but this is a level of incompetence I've not seen in 20 years of online games.