Since the last major patch from 2 weeks ago there was (and maybe still is) a memory leak affecting the crowded areas like mushins tower where a lot of characters gather. You dont need to be a genious to see it happening. Its not the first time this happens and it takes time to fix since they need to figure out why its happening. Give them time, until then, dont stay long in those areas.
To the people who say its a bad server issue: its not! we run on the same server (with later on them being modified for better performance) as we had back in the release days with millions of people playing. Never trust an engineer who knows nothing about software :D even if he works for NASA. Back in the old days the servers were way more crowded.
Current FPS issues are caused by (I will not put my head under this one, its only speculations) UE4 textures running on UE3 environment, its been like this since they made that change a few years ago. The engine doesnt care what kind of RIG you have, it runs on a single core with little to no rendering from the GPU side. If your CPU have integrated GPU, good for you, it will run smoother, if not, you are screwed. Thats how UE3 works. UE4 on the other side moves everything out of the CPU loading heavily your GPU. Will it be better? Maybe. Will it be worse? Highly doubt it.
MS lag is entirely on your ISP routing to the server location (Frankfurt for EU and idk where in Texas the US server was :D ). I live 2000+ km (sorry, no american measures here) away from the EU server and I have 80-130 ingame ping. How to fix that? Move closer to the server or use VPN for better routing.
And just to add up for the people that are going to ask me or blame me for whiteknighting, I have 15-20fps in raids, 40-60fps in dungeons with no crashes (other than afking in the memory leaking areas since my RAM overloads and the game crashes on its own), other than the normal fps drops in raids and such. Running on i5 7400, GTX1050TI with 16GB of DDR4 2400MHz and the game itself on an Intel SSD. Game running on 2 or 3 graphics settings with ingame limiting the FPS to 60 (you dont really need more, yes I can run 120 but whats the point anyways since it drops to 20 in raids whatever you do). Some other options ingame can make your game render less which means less fps drops, but find that out by yourselves.