DevilutionX now runs the render at the monitor refresh rate.
I saw this update the other week but haven't yet gotten around to trying it out.
So, how does it work exactly? I imagine that the game itself must still be using some kind of 20 tics/timer so that the gameplay doesn't get sped up or otherwise adversely affected, whilst outputting more frames for the graphics card to display. Also, I would expect that the higher framerate would only really be noticeable for the map scrolling and the mouse pointer as it wouldn't add any additional animation frames for the sprites themselves.
How much of a difference does it make?
The game loops renders the screen every frame but only does logic at 20hz, this isn't much unlike how many more modern games where the rendering and logic is fully decoupled (not quite the case yet).
The world still scrolls the same (for now). It does make the mouse a lot nicer to work with. Also, transitions and dialog text is now smoother. It also makes it easier to find items on the ground as the mouse doesn't make big jumps when moved quickly. Sprites (like fireballs) could also be made to move more smoothly across the screen.