I suggest that building speed should be decreased pretty much (f.e. at least x3x5 times) and it should take even more time for some constructions in towns and castles to be built.
Building also shouldn't be free. There are some projects, that may be free or cost practically nothing, but these should maybe be only a few of 1 tier.
And increasing speed x2 should cost more than 500 gold a day, so that you would think twice before doing so.
Right now building a fortune is not very hard. So at least it may also balance economy a little, not saying about it's way too unrealistic to build most of constructions in 1-3 ingame days.
I'm aware that in Bannerlord a year consists of 84 days, so 1 Bannerlord day should be considered as 4-5 days. And it's still too easy and fast to build every city project.
Buildings constantly need to be maintained with little money. The higher the tier, the more money. If you don't have money, there is a bar of building decaying. So you have time to get money. Not like one day, a long process too.
Also after sieges some buildings can be damaged and you need a little money and time to reestablish them.
It would also be nice to be able to upgrade your villages in some way.
I understand that it's a game, but still the building is too fast.
UPD. My other suggestions connected with this thread one way or another.
Build your own fief with future upgrades. Detailed.
Capitals/all fiefs with unique buildings and additional opportunities.
UPD. Some of my other threads, which are not exactly connected with this topic, but can make this game better in one way or another:
Suggestion. Make training grounds the new main source of training, especially for recruits. Add recruiters and patrols in the game.
Suggestion. Building in towns and castles is way too fast and free. Maintenance.
Tools to balance food and prosperity. Detailed suggestions for food shortages.
Suggestions. Maths and Winter attrition for balance. Detailed explanation
Hideout difficulty. A possible solution, both if you like or hate them. Detailed.
Mod slightly connected with the thread. A tool to invest in villages.