Hi Everyone,
My first post on the forums: what a great game this is!
I’ve been building a bit around and encountered an ugly bug.
When i’m building a multi-floor building (4 floors), the lua lag is tremendous (like, 99,9%) as soon the Hearthlings start to work above the second floor.
Log is included here. 
(edit) Interesting is that the lag disappears instantly when the hearthlings do something different than building on the subject building. the lag/freeze reappears when the build continues.
This is a bug that’s been here since multi-story buildings have been introduced. I would suggest to simply avoid making buildings above 2 floors or use the ib command to automatically build them.
To use the ib command press the ~/` key to open up the console (normally found next to your number key row).
Select the ghost building (or once you’ve finished designing it you can just open up the console
Type in “ib” no quotation marks.
Press enter.
Tada! Your building is automatically built, costing no resources.
That’s great! Thanks!