I am using a MacBook Pro 15" (2018) with 32GByte RAM. I do not allow catalogs to grow over 30,000 images before I split them. Beyond that, C1 really feels slow. The magic threshold seems to be somewhere between 20,000 and 30,000 images. For example, you will start to notice the time that is required to open or close the catalog as annoying. I have found catalog sizes beyond 30,000 images unbearable.
Your experience may vary depending on how you use the application, though. For example, Catalog Collections > All Images is basically unusable with larger catalogs (even if they contain less than 20,000 images). If you're a wedding shooter that wants to put all 5000 images into one album you might run into problems far earlier. The maximum number of images might also decrease depending on how many edits and layers there are.
Because you mentioned performance: Editing is really slow on LG 5K screens. Drawing masks with the brush is more like hit and miss. That works a lot better if you're only using the internal screen.