Unless there is a lot of good evidence, I am skeptical of this claim

Human children are exposed verbally to 1x human speech. Do we really think that making teachers talk faster will improve learning and retention?

Also, according to linguistics, I believe pretty much human languages transmit close to the same bit rate (some languages have longer more descriptive words, some have shorter words, but by and large they average out).

Throughout our evolution, we have been exposed to 1x speech.

My guess would be that are brains don’t have a learning block to 1x speech.