First, it doesn’t matter that it’s not playing. The audio engine is working even when it’s stopped. And, there’s nothing wrong with your system.
Here’s what’s going on…
The higher CPU load with Record Enabled is normal and how ASIO Guard 2.0 works. Just leave Record Enabled off on all the channels (except for the one you’re currently recording on).
Since ASIO Guard is a dual buffer system (a large buffer and small one for low-latency stuff) that smaller buffer (for Recording at low latency) is the weakest link in the ASIO-Performance-Chain, as far as the meter is concerned.
You’re seeing your CPU try to crunch 4 VSTi’s into that smaller “Record Enabled” buffer.
To make matters worse, my guess is that the smaller buffer used for low-latency recording has to “fit” all on one CPU core (you have 6 cores).
Another way of saying this: my guess is that only the larger buffer (non-Record Enabled tracks with ASIO Guard on) gets to enjoy splitting up the audio workload across cores (a.k.a. multicore “audio scheduling” is the technical name for it). This is how Sonar’s audio engine works.
So it’s many things working against you:
The smaller “Record Enabled” buffer (with 4 channels trying to squeeze into it).
The strong likelihood (based on the evidence) that Cubase has to do all that work on a single core, whereas when you uncheck Record Enable for a given channel, it can not only shift to the larger buffer, but shift across other cores.
The fact that Intel CPUs with higher core counts (6, 8, etc.) tend to see lower performance per-core, but greater overall performance due to the extra cores. So, if my suspicion that #2 is correct (and I think it is), then you’re trying to squeeze those 4 channels on a single, lower performing core (compared to cheaper 4 core CPUs), at that. So the premium 6 core CPU may be hurting you a bit for this use-case. A cheaper, 4 core of the same generation and same clock speed may have just enough more performance per-core, to pull it off (or not).
Overclocking would probably solve the issue then, because you just have to get a little more juice out of one core.
But, if you can, just disable those Rec buttons (except one). Problem solved.
(There’s a pref setting to turn off the auto-Record-Enable that will make this easier.)