For future reference, Leap 15.1 VBox 6.0.12 VM scale 200% guest 100% is very laggy with new VboxSVGA driver and 3D acceleration enabled. This does not occur with the older VboxVGA driver, but it will be deprecated. The third available option, VMware’s VMSVGA, does not autoscale.
So for now it seems better to run VboxSVGA with 3D accel disabled.