As far as I know, there's only one cause for this problem, which is a "bug" in the vanilla game, though I may be wrong. My understanding is that basically, on computers with more than one graphics processor option, Noita will choose one to use as the default for running the game. So for example, My PC has "Integrated Graphics" as an option in addition to my actual graphics processor. Noita chose Integrated Graphics as the default, which doesn't work as well, so when the graphics processor is overwhelmed by whatever the game is simulating, ie fire particles or lighting effects, it will slow down the game to let itself catch up. And it does seem like the outdoor area is more demanding, so it'll naturally slow down the game more.

The fix that I found via other people having this issue is: Open the game's root folder, which if you do not know can be accessed from the game library by right clicking Noita, hovering over "Manage", and clicking on "Browse local files". If that isn't working for you, you can try to navigate to it manually in File Explorer, the path should be something like this:

C:\Program Files (x86)\Steam\steamapps\common\Noita

Once you are in the Noita folder, right-click on "noita.exe", or just "noita" if you do not have file extensions visible, go to "Run with graphics processor", and from there you should see what options there are for the graphics processor, one should say "(default)" on it, this is the one Noita is using when you launch the game through Steam. If you click on a different option (there should only be one other option, your actual processor), Noita will launch using that one instead, which should fix the problem. However, you will need to launch it this way every time, or change your default graphics processor, which there is a button for in the same place (Run with graphics processor), but the button did nothing when I clicked it, so idk if it'll work for you or not, and I don't currently know how else to change the default so you're on your own there unless someone else pipes in.

As for the connection between this mod and the slowdown, it's probably because this mod adds extra content and possibly scripts to the game, making it run slightly slower. So you probably already had this bug, but your PC was just barely handling the game well enough to play at full speed, and this mod was the straw that broke the camel's back, as they say. Hopefully this solves your issue, or at least points you in the right direction. Good luck!

@Krugeren, Coudn't have said it better myself! This is an excellent explanation, and should hopefully clear things up.