Everything slows down.

[Angelhearth]

My game was going slower and slower, and, to my horror, I found that it was because of all the windows I kept open. I need those windows.

Town Hall (for up to date information on items)
Inn's (2) to see how many are there, and when I need to start building more houses.
Markets (2) to see if they have enough manpower, and inventory.
Schools, Churches, Graveyards, Hospitals etc, to keep an eye on things.

All of these obviously updates way, way, way too often, slowing the game speed considerable (I'm talking X10 being slower than X1 with these windows closed).

What I need is a window where I can watch the essentials. How much Copper Ore, Iron Ore, Tin Ore, Hardwood, Clay etc do I have, at all times, how many people are staying at the inns, stuff like that. Has anyone made a mod where you can choose what items to monitor?

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Here's an image showing what I like to have open.

[Vrayna]

The only way to keep an eye on exactly what you have is sadly the townhall's inventory window, which indeed slows things down.
There's a few "mitigation" things you can do:

Keep as few inventory window open as possible. Close those market windows, and put your townhall window on the Trade Items tab so it stays open and you can have a quick look at the inventory now and then, without having it constantly updating numbers. In the general info window, you also have the number of homeless people shown. Just based on that, I'd say you don't need the inns opened either (though I don't think thoe contribute much to the "lag").

Reduce game speed (will help the game assign jobs/pathing in time, before it needs to recalculate everything for the next "round" too). At 300+ population, you might start running into issues with people nhot doing anything otherwise.

Restart your game regularly. Cached stuff doesn't get cleared correctly so the game will slow down during long game sessions. The higher the population, the more you'll notice that issue. So restart every couple of hours at least.