What the SloMod does (and what I also did, although not in as drastical a way - 2 turns per year is too slow for me) is modify data\world\base\descr_regions.txt

A region entry looks like this :

Inverness_Province
Inverness
scotland
English_Rebels
20 25 225
atlantic, explorers_guild
5
4
religions { catholic 90 orthodox 0 islam 0 pagan 5 heretic 5 }

The bold number is the province's farm output, which affects pop growth and crops cash.

Since I decided to play 1 turn / 1 year, I just halved all values, rounding down. Now, most regions not intended to have fertile farms and fast pop growth (like Tuscany or Egypt) will actually have the pop die out if I don't build at least basic farms (which may be a bit of a problem with settlement held by rebs for too long, I guess...). Oh, and I'm definitely strapped for cash until I get to either farm 3, or massively devellop trade and merchants, meaning tech upgrades also get slower.

All in all, I'm pretty happy with the results.