YES/NO Midi track itself hardly use resources. It’s the VSTi connected that uses much more.
Again it’s the VSTi using resourses. A midi track with no link (deleted VSTi) uses virtually nothing.
Everything that plays and produces/calculates data uses CPU. Even midi tracks, although this is neglect-ably small compared to instrument tracks. VSTi uses lots more resources but it strongly depends on what VSTi is used and how the used patch is build. A patch that uses a single oscillator to generate a simple sine wave with maybe some basic filters uses almost nothing compared to one with 4 oscillators using granular synthesis with morphing. Also some VSTi’s are better written than others and use fewer resources when producing the same type of sound.
And don’t rule out your inserts and send effects. Also these can put a lot of stress on your CPU!
It helps to use VST 3 in stead of 2.4 if possible because you can specify these to automatically render inactive when not actively used. They will only become active when receiving events.
It might be enabled by default with the latest versions, but I’m actually not sure about this? Anyhow, you can enable/disable this under Preferences -> VST -> VST- Plug-ins -> Suspend VST 3 plug-in processing when no audio signals are received.
Additionally you can also experiment with ‘Asio Guard’ under the device settings of your audio interface and see if this improves performance.