Compositor-level Black-Frame-Insertion
Feature summary
Black frame insertion is a method to reduce persistence blur that occurs naturally in the human eye. This has historically been achieved via PC-Monitor firmware, either by replacing every second frame with a black frame, or when the monitor hardware supports it, by strobing the backlight of the monitor in sync with the Monitor drawing the screen. (See https://blurbusters.com/faq/motion-blur-reduction/ )
On high-refresh-rate monitors, which are becoming ever more popular with consumers, this can make an incredible difference to the perceived motion clarity. Unfortunately many monitor manufacturers refuse to implement this feature for one reason or another, and the same goes for GPU manufacturers (with the limited exception of Nvidia, which introduced "ULMB" for Monitors with their G-SYNC FPGAs). Particularly in game emulation software this effect, when implemented in software, has shown to be effective in restoring the perceived motion clarity that CRT Monitors used to have.
This feature is in my opinion best placed in the compositor, as the compositor is what's closest to the vsync interval other than the GPU drivers themselves. Because in a software implementation these strobes have to come perfectly timed to stay inside of the vsync interval, it's crucial that the "topmost" system controls the black frames.
How would you like it to work
Since Mutter is always run in vsync, it's easy to draw a black frame in the back buffer every other frame. But there should also be ways to change the target ratio of drawn to black frames. While Mutter would have to continue drawing synchronously with the monitor, client applications could be synced to a lower ratio, based on the black frames. Example 240hz Monitor with 50:50 BFI, Mutter draws to the framebuffer at 240 frames per second, but to client applications it signals that vsync is actually 120hz, to which the clients can sync potentially allowing for energy savings as they now potentially have a longer waiting interval.
A caveat is unfortunately that this will never reliably come from even higher in the stack (GPU driver or Monitor Firmware). GPU manufacturers hate consumer-grade customers, as all of the money is in corporate customers, giving them no incentive to work on this. Monitor manufacturers are also not incentivized to add this feature to more monitors, as they of course want their customers to pay for the more expensive monitors in their line-up. The only places this can realistically come from is from client software or the compositor.