No, we spent a lot of time optimizing. Especially level design. At some point though, you can not strip down anymore and have to rely on multi-threading. It is what any modern game does.
Try yourself with my test case suggestions from above.
If UE4 is poorly using remaining cpu power if frame rate drops (due to cpu bottleneck), then it is an engine problem. There is nothing you can do to solve poor multithreading with level design etc.
You will see.