No, it's not a performance issue.
The Steamdeck lets you open an overlay that gives you the details of exactly what your CPU and GPU are doing.. Eventually, the game will just decide to lock itself to 24 FPS, no matter how little the GPU or CPU are being used.
I could stand in an open field, with every detail set to 0, rendering at 25% resolution at 800x600, with an uncapped framerate, and still only 24 FPS. While a new save in the same spot with the same setting can get well into the 100s.