I've been playing this game for almost 2 years and I can tell you that there are two major issues with performance that happen whether you're on a high end desktop PC, or something less.

1. Performance is generally very poor in this game. It's most apparent when you're on a high end gaming PC (like you have) because the performance is basically the same no matter which graphics settings you change (because your PC can handle the graphics just fine). Some people will talk about how "CPU intensive" this game is. I don't really buy it. On my high end desktop PC, none of my cores is even close to 100% utilization when the game is bogged down. Which brings us to ...

2. The biggest issue with performance is either server performance or networking (no real way to tell unless you work at the developer). How does this manifest? Sometimes your inputs don't translate into actions on the screen. Sometimes they happen much later than when you sent the commands. Sometimes you can't weave light attacks between abilities. The game just feels like it's running at 5 FPS, even though your frame rate counter displays a normal frame rate.

Watch streamers on Twitch play the game and list to people say things like, "I'm going to need to reload UI," "I need to reset my game", "We've been in here for a while and the instance is starting to bug. We should reset the instance." These are pretty good indications that there are major problems server side (multiple people suddenly having performance issues at the same time). I'm not sure I've ever seen an end-game raid be streamed without people saying things like that. I'm sure the devs know about it, even though they don't publicly acknowledge it. There are probably people working on the problem full time, but just understand that performance issues have been going on for years, so don't bet everything on some big patch coming that fixes it.

By all means, fiddle with INI settings, graphics settings, registry edits, add-ons that adaptively change graphics settings, etc. I hope you get it all sorted.

"They're not all running the skeleton polymorph because they like the way it looks. They think it helps performance somehow"

The theory is that the skele polymorph is a single bitmap, rather than lots of different bitmaps from players typical outfits, and therefore reduces load on the client by not having to render so many different things.