When fixtures of different beam types are set to an non-zero intensity at the same time, the 3D window tanks.
For example, I have a fresh show file with an x4 bar using the "Wash" beam type and a JDC-1 with the "Rectangle" beam type placed next to each other.
As shown in the pictures below, when either fixture is turned on independently, it works fine showing 63 FPS in the upper right corner.
However, turning both fixtures on at the same time causes 3D performance to significantly drop down to 3 FPS.
This was observed while running onPC with v18.104.22.168 and previous versions as well. I'm not able to test this with other hardware as I don't have access to them.
Turning on 3D Priority has the same behavior.
This behavior exists in all BeamQuality modes except None and Line.
I've also observed that while both fixtures are on, GPU utilization is LESS than idle utilization with both turned off but 3D window still visible.