Does the game run at the same frame rate in both situations? It almost sounds like you're using a different power plan where your CPU is bottlenecked in one mode but not the other.
All a bit odd, the "Performance/Economy" toggle is mainly aimed at laptops which could run the game on either a discrete graphics device or an integrated one. On my system, with only one graphics adapter, it won't even stay set to "Let windows decide".
Warframe is a multithreaded game, typically it will run 2 primary threads very hard and then some other auxiliary tasks. So the idea that it can't use multiple cores is incorrect. Something to keep in mind however: Windows task manager can be very misleading for identifying CPU bottlenecks. Modern machines are designed to move threads between cores, in an effort to distribute the heat output. Task Manager is bad at showing this because its fastest polling interval is longer than the time which threads dwell on cores. So what looks like multiple cores having very low load, is in fact a small number of threads that are running full bore but getting moved around. If a thread runs 250ms on each of cores 0, 2, 4 and 6, then Task Manager's 1 second polling interval will show that on the graph as 25% usage of 4 cores at the same time, not, 100% of a single core one after the other.
The real proof of how well the game runs is... how well the game runs 😁
Are any of you overclocking, forcing process affinity, or have you disabled multithreading, or disabled one or more CPU cores for your machine?
So, for DX11 mode, normally the video driver will build a cache of all the graphical shaders that a level will use. A huge amount of CPU usage is a one-time thing where your video driver will compile all the shaders, and then put them in the cache so that it doesn't need to do it again next time you play. If you're seeing this happen repeatedly, then it probably means that your shader cache isn't working correctly.
Check your GPU control panel and make sure that you have not disabled the shader cache nor constrained it. This should be on Default unless you have very specific reasons not to. Likewise make sure that your storage device is not dangerously full.
Your shader cache for warframe might also be corrupted. You could try deleting the Nvidia shader cache and then running the game normally. In this scenario we would expect very high CPU utilization the first time you load into a level, but only the first time.
Failing that... realistically speaking DX12 is the future and runs much better on modern hardware! If DX12 works great for you and DX11 does not, then seems like a great time to switch.
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