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