Don't know if it's been mentionned already (didn't read the whole post, once it started drifting into a "people who change the scale shouldn't" VS "people do what they dang well please" debate, I got bored quick ) but there's an elegant way to solve both building "times" and pop growth in one swoop : divide all provinces' agricultural values by the same factor you've divided the turns by.

Now, I can't remember which file to edit and which of the (uncommented) values it was you had to tweak but I could find it back, I know I've made a couple posts about this back in December when I was still active here... (haven't reinstalled/patched/unpacked M2TW yet, I'm still waiting for more feedback on 1.2, especially on the traits front which was my major beef with 1.1...)

Anyway, the point of it is :
- it makes a lot of provinces have a negative pop growth value early on (meaning you *have* to build farms and then wait if you intend to move past "dirt heap" tech level),
- Even when fully teched up, towns and castles grow very slowly, though good governors can help with this - slower gameplay
- you have a *lot* less cash flowing around meaning you can't build everything everywhere and have to make choices. That cathedral might still only take 6 years to build, but if you have to put it off for 20 years, the time it takes you to painfully scrape enough cash to do so, then it has a "realistic" time frame.

The only problem with that solution is that trade rates and mines are not affected (hard-coded on the 2yr/t values), which makes sea commerce somewhat overpowered (and merchants too, to a point). I'm sure one could play by house rules to mitigate this, as I did back then.