Seeing how there are lots of other posts on this, I just wanted to throw in my extra info: I have no trouble running any other game on my system, when I can actually load into a mission performance is flawless, and CPU usage goes from ~10% to over 70% whenever I try to load into an instance; if I can successfully enter anything, CPU usage goes right back down to the previous 10-ish percent. Most of the time it takes so long to load that it won't even let me load into a mission. I basically haven't touched the game since Duviri launched (outside of testing to see if the game works again, which it still doesn't) because of how it's borderline impossible to actually enter an instance anymore.
I have tried optimizing the download cache, I have tried completely reinstalling the game, I have tried reinstalling the game on a separate hard drive, I have tried purging all the residual files from previous installs, etc. Nothing has helped these awful load times.
Also worth mentioning is that if I can actually enter a game, I can actually be playing multiple other games at the same time while maintaining 60fps in Warframe with no stuttering or other issues, just like I had been able to before Duviri. This is not a matter of my PC not having the power. They did something to how the game loads missions that absolutely tanked performance.