Do we know why Fractional Scaling limits FPS in games?

I have a 120Hz monitor with a PC loaded with an nVidia RTX 4090 and a Ryzen 5950x and am using Zorin OS 17. The default scaling is naturally 100%, which renders icons and text way too small on my display. 200% is way too big. So I enabled fractional scaling and 150% is just right. I also set my display to the correct 120Hz (119.88 Hz to be precise). So I then set up Steam and downloaded a few modern games (Starfield and Baldur's Gate 3, to be precise). I noticed with VSync turned on both games were capped at 60Hz...

So I spent several hours trying to figure out why the games were capped. I tried everything; tinkering with Lutris, Proton-GE, NVidia drivers, various launch commands, x11 config, AMD Freesync, NVidia G-Sync (my monitor is capable of both)... nothing was working. I tried turning off VSync in the game, and the framerates increased sharply, but the games turned into a stuttering, screen-tearing mess.

Hours in, and it never even occurred to me that fractional scaling might be the issue. But I thought what the hec, so I turned it off and VOILA! The skies opened up, the world became beautiful again, and my games were finally rendering beautifully and smoothly at full 120fps, no longer capped at 60.

So my question to those of you who know MUCH more about this than I do... why does fractional scaling limit games to 60fps? And also is there a workaround? Thank you so much! This is actually my first post here. I'm loving Zorin thus far, as it is one of the most impressively full-featured, "it just works" distros on the market (and believe me I have tried most of them).

Fractional scaling calculates the rendering of the desktop using a non-integer scale factor and then scaling it to fit the screen resolution. This involves a great deal more calculation than a non-scaled display.
This gives Zorin OS Lite a distinct advantage in using DPI instead of Fractional Scaling.
You might try adjusting games internal settings to adjust for performance over quality...
This would be limited, but may be enough to get you over the barrier of hefty calculations being run and help spare the CPU from heating, as well.

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Thank you for the reply. I had a feeling it might be related to this, since so far as I understand fractional scaling works differently here than it does on, say, Windows which simply commands apps to run directly at the fractional scale rather than running at 200 and downscaling to 150, for example.

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