Windows 7 user here.
Using the old launcher.
I logged in today, first time in a week, and right on the start menu with the pin code i got a slide show with about 5 fps. After some alt+tabing and looking on the temperature of my CPU/GPU (which was all cold as ice) the fps returned to it's normal 120. But when i got to the character screen it again dropped. After some alt+tabing it went back to 120 again, but after loading in to my character it droped back to 4-6 fps. After this i could return my normal fps with some alt+tabing, but the moment i open my inventory, quest menu, bank, skills or anything - it drops to 4-6 fps until i alt+tab for some more... This is ridiculous.
People in chat told me that some of them had simillar problem with the after they installed the new launcher, but i still got the old one, so it's definately not a launcher problem, but most likely - broken game update or broken anticheat update.