Compiz uses a lot of CPU when GPU acceleration isn't available (e.g. in a VM)

Compiz uses a lot of CPU when GPU acceleration isn't available.

How to reproduce:
Set up a VM running 16.04 with non-accelerated graphics and 1 virtual CPU (I use VMware Workstation and unselect the 'Accelerate 3D graphics' tick box under Virtual Machine Settings -> Display; glxinfo |grep renderer shows "Gallium 0.4 on llvmpipe (LLVM 3.8, 256 bits)")

Method #1:
Install indicator-multiload and configure it to refresh every 100ms

On my Core i5 4200 laptop, when I set the VM resolution to 996*674 px, compiz generates ~30% CPU load.
When the VM resolution is set to 1024*768, compiz generates ~70% CPU load.

Method #2:
Start top in a terminal and open System Settings: things are normal, compiz CPU usage is ~1%.
Select Displays: compiz CPU usage jumps to ~65%, and stays there forever.
When adding a 2nd virtual core, compiz CPU usage jumps to ~115% and stays there forever.