I am concerned that there might be something in VT that are causing high power consumption.
At login prompt FreeBSD 12-CURRENT on my UP board (Intel Cherryview) the power consumption is steady at 5W when idle, measured with a watt meter on the 120V side.
When I do kldload i915kms the power consumption drops to 4W. As comparison, even running a simple OpenGL app with 60 fps refresh rate in KMS mode does not cause higher consumption than 4-5W. Starting X brings it up to around ~5W again.
In /boot/loader.conf I'm loading
CPU is 99.8% idle when running top in from ssh login.
The watt meter only show whole watts so the difference might not be as big as 1 watt but there definitely is a difference. As someone else said in the mailing list, the difference is so big in their system it causes the fans to run high and changing to syscons solved the problem.
I finally took some time to work on vt(4) performance. My concern was around the input performance (how fast we can write to the console). I didn't do any power consumption measurements, but the patch might help it.
If you have time, could you please try it?
D15302 has been committed