Another closely related possibility is that a lot of processes in science are inherently serial, i.e. they can't scale with the number of researchers, because they require slowly integrating information and making consecutive steps of progress.

Maybe after say, quantum computers are available, it takes around 25 years for a team of researchers to slowly make progress and integrate information until they find a killer application. Funding 25 teams of researchers may make them pump out papers 25x faster, but they aren't going to find the killer application in 1 year.