I might not followed this completly.
Is the issue that the CPU is using 100% or that the FPS is low. Regarding why performance has decreased I can’t answer that at all but I might explain why 100% might be reasonable for one core.
But regarding 100% uthilization I would assume this is expected when you are CPU bottlenecked. Which VT2 oftens is due to heavy CPU usage.
The model is that we have 2 main threads, game and render thread that are the heavy lifters. We then have worker threads. If one of the main threads is the bottlneck it will likely be working ~100% of the time unless you cap the frame rate. if you would be GPU bottlenecked, these 2 threads would be idle in the end of a frame and thus not be working 100% of the time.