Considering Aleph and ES seem to be going for an aversion of the Earth being *Super Special*, probably not exceptionally fast?
I think that DS civilization actually develops faster, seeing as how they have higher standards of living than first world countries on Mid and other TSAB core worlds after multiple apocalypses. The surprise would probably come from how high civilization can develop in the absence of magic. It would be like a Flintstones or steampunk world. The large population is just a natural consequence of advanced civilization + no dimensional emigration + no apocalypses + Type I planet (ideal habitat, native human civilzation, long history)
Now I'm fairly certain that this isn't just my headcanon and that there were some posts mentioning this, but Earth actually has a *Super Special* backstory! UA-97 is actually a sister dimension to Al-Hazred which budded off in a dimensional splitting event about 70,000 years ago (coinciding with irl genetic bottleneck and Lake Toba supervolcano eruption) and on one side, all the mages were ded and on the other they survived. I'm not sure whether this was WoG or WMG though.
Of course something like this isn't going to totally extirpate magic genes from the population, but would have wiped out 100,000 + years worth of magic tech tree advancement, at the early bits where it is the slowest. I also like to think that Earth mages are rare because our genepool hasn't been infested with Al-hazredian engineered gene variants.
Now I'm actually really curious about Akkamar's backstory. I mean, they were probably like a small outpost on an incompletely terraformed deathworld and they somehow survived and rebuilt their civilization?