I think there are two separate things. Slowness of progress in research is good bc it signals high value/difficulty. This I wholeheartedly agree. The other is, the slowness of solving a given problem is good, which is less clear.
I think indubitably intelligence should be linked to speed. If you can since everything faster I think smarter is a correct label. What I also think is true is that slowness can be a virtue in solving problems for a person and as a strategy. But this is usually because fast strategies rely on priors/assumptions and ideas which generalize poorly; and often more general and asymptotically faster algorithms are slower when tested on a limited set or on a difficulty level which is too low
I haven’t looked into the source study, so who knows if it’s good, but I recall this article about smart people taking longer to provide answers to hard problems because they take more into consideration, but are much more likely to be correct.
https://bigthink.com/neuropsych/intelligent-people-slower-so...
I think part of the message is that speed isn't a free lunch. If an intelligence can solve "legible" problems quickly, it's symptomatic of a specific adaption for identifying short paths.
So when you factor speed into tests, you're systematically filtering for intelligences that are biased to avoid novelty. Then if someone is slow to solve the same problems, it's actually a signal that they have the opposite bias, to consider more paths.
IMO the thing being measured by intelligence tests is something closer to "power" or "competitive advantage".