Don't look at task manager to evaluate CPU utilization, i don't think it's very accurate at all. Out of the interest i run short test with various thread override settings to see if it has any effect. At default settings -1 which is 31 threads for my CPU, the test was completed in 61s, at 24 threads the time increased to 109s, at 16 threads it was 149s and at 8 threads - 269s. So the graph might not be linear, but threads restriction is clearly working and interestingly the task manager in all cases but the 8 threads run, was showing 100% CPU utilization. At 8 threads it was showing 80-90% utilization. I also tried to set threads override to 0 to see if that will have any effect, but there was no meaningful improvement - 59s. By the way viewports interaction remain the same at any settings, which is relatively smooth for me. If you have very high threads count CPU, your experience might be very different, but the main point of my post is that Windows task manager is probably not the right tool to evaluate your CPU utilization during interactive rendering.
