We do know that response to difficulty influences performance in iq testing: https://pmc.ncbi.nlm.nih.gov/articles/PMC10990577/

This study found that adding or removing rewards for performing well can pretty dramatically impact performance.

"it is unclear to what extent the positive manifold reported in intelligence research since Spearman (1904) might be explained not through a shared component of intellectual capacity, but through a shared component of effort or time investment in testing tasks."

So, yes, we don't know, but the ways we don't know should also include "we don't know if iq testing is even measuring intelligence rather than stick-to-it-ness".

I mean I agree with that too! I would just bucket all this under "we don't know". :)