After updating my macOS, Parallels 12 has broken loading a VM from the Bootcamp partition. This has forced me to use my Windows 7 virtual machine and I have discovered a weird problem:
The mouse cursor inside the virtual machine is really slow if an application is loading something or doing some activity.
At first I thought this was because the machine was hogged, but the problem is actually simpler than that: Windows 7, in Parallels 12, is really slow when rendering an animated cursor. I have changed my cursor theme to an old non-aero one and everything is fine now.