Rack runs both the CPU and GPU on a hot loop, throttled only by VSync and if your audio output plugins are connected and stalling the compute cycle. So yes, it’s normal. Try @JimT’s soft-
fork and set a low but still interactive (ex. 30FPS) rate and your computer should calm down somewhat.
Normal DAWs sit idly and do nothing unless they are told explicitly to do something (ex. you hit Play.) Rack is always running the entire cabinet (unless you change the engine sample rate to “pause.”) They also operate in audio blocks, either from the audio interface asking for samples (where they then run a DSP pass, hand over the blocks and then go back to idling) where Rack is permanently stepping modules a sample at a time. Another difference is that plugins tend to be slept when they are not in use, either automatically or by user option (ex. in REAPER you can “freeze” a track which turns off the FX chain and inserts a new track playing a static sample.)
So the short answer is yes, Rack will ultimately use more compute than special purpose tools. It probably will also remain more flexible than special purpose tools.
Multi-threading using more or less resources depends; there is a wisdom in some circles that a CPU which is fully utilized for a short time and returns to idle will use less wattage than a CPU which is partially utilized for a longer period of time. This entirely depends on the computer though, since those measurements are based on ex. mobile processors that are constantly scaling up and down their power, and were designed power conscientiously. If you’re running that 40-core Xeon madness someone else was, it’s probably not likely that you’ll ever save any energy no matter what strategy you do (but I do not work closely with Xeons, so maybe they do have great power scaling and I just don’t know it.)
That may change easily 
Granted my laptop is not too many years old yet, but draw calls seem to be the #1 bottleneck (it still runs interactively off the intel integrated.)