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 ...
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