It would make sense if situations where the native panel resolution doesn't match the framebuffer resolution are causing the driver stack to have to maintain two framebuffers (native and non-native) to do the upscale and then feed it to the monitor. GeForce drivers on windows specifically have a setting to control whether this happens or not (let the monitor scale it, have the GPU scale it). The upscale would not only use more VRAM in that case (and as a result more memory bandwidth) but it'd need a bit more GPU compute as well to perform the scaling.

On my low-spec laptop from a while back, running Rising Thunder set to 720p was faster than native, but it was even faster to first set my desktop resolution to 720p because the overhead of the game engine scaling its 720p framebuffer up to native 4k was measurable. Setting the desktop resolution down also improved battery life.