I've had many cases when lowering the resolution didn't increase my FPS by more than like 5% when I was CPU limited. This might be the case here.
In the now-deleted tweet, the developer (who's actually a character technical artist at Rocksteady, not WB Games Montreal) mentioned the GPU being the issue. It's definitely nowhere near the capabilities of that in the Series X or PS5:
FP32 max throughput (XBSS vs XBSX vs PS5 - TFLOPS)
4 vs 12 vs 10
Texture fillrate (GTexels/s)
125 vs 380 vs 322
Pixel fillrate (GPixels/s)
50 vs 117 vs 143
Primary memory bandwidth (GB/s)
224 vs 560 vs 448
I also suspect that the triangle setup rate in the Series S is much lower, probably around half, than the Series X. But that said, Fleur Marcy (Exec Producter at WB Games) did specifically say that "
it's not as straightforward as lowering the resolution and getting higher FPS".
Given that there isn't much difference in the CPUs, but there is with the GPU and memory sub-systems, I suspect that it's a combination of things like triangle setup and memory read/writes that's the issue here, which would tie in with her comments about the focus on the game being a 'f
ully untethered co-op open world.'