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 😁
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