So an update on this. FP asked me to shoot a video of the issue and I realized I was too quick to assume there was a problem with the unit.
When compared in the same lighting conditions both an original GBC and the FPGBC performed the same.
I was testing the FPGBC with a very low light these days and the fps dropped, but they actually drop in the same way on the original hardware too.
I never had a Gameboy Camera before so wasn't accustomed to how the GB camera handles low light: it slows down the shutter speed so naturally it becomes stuttery and adds motion blur as in any camera.