I don't see what you see though... less pop means that you don't get to use as many new laboratories, that is my whole argument that less pop give you less actual research. The same should be true for Intelligent trait for example, these a percentage based so will effect everyone equally harsh, the cost to build a laboratory will have no direct impact on the difference here.
But having less POP means that the techs are cheaper, meaning you don't need as many new laboratories as a "wide" nation to research them, so increasing buildcost would not affect everyone equally harsh.
Intelligent, natural physicists/sociologists/engineers traits and the materialist ethos all increase
tile output of "science resources", meaning in that timespan of the game where you can't build laboratories they would have a significantly higher (specifically +20%/+20%/+35% in any order) output of "science" than everyone else for much longer, which would cause snowballing with those traits to be much easier.
Especially in the mid-late game if you choose to go for a "tall" empire instead of a "wide", since everyone else gets an increasing "research speed" penalty in addition to not being able to build/upgrade as many research stations and laboratories as they normally could to compensate.
It's not that the immediate impact is ridiculously large, it's the fact that the potential snowballing is incredibly OP.
You will get more tech in the beginning and less as time go by in comparison with the vanilla game, but that would be a conscious choice. The whole idea here would be that the increase in science are significantly reduced while being roughly the same in the beginning so you can still get a decent amount of early tech while by mid game tech progression will slow down considerable and slightly pick up again at the end of the game. You simply get a different curve and primarily get the slower progression when you want it the most as a design choice.
Exactly what you prefer then is up for discussion though.
Well i do agree that it is a subjective preference and a design choice how you want the "curve" to progress, it still has to be balanced so that no specific playstyle is overtly broken.
(Also most of my responses here is not really indicative of personal preference, just offering some tips based on my modding experience, and the thread name being "slowing it
all down" might have had an impact on my perception of OP's vision for their mod-to-be

)