If it slows down depends on you.
Things that may cause calibre to slow down are:
- Complex custom columns that depends on other columns, especially if you sort on them.
- The presence of a column with the available formats. Calibre will under some circumstances check the actual files in the library to see what those formats are, and that can take a long time.
- Displaying the tags browser when you don't actually use it.
If you avoid these issues you should be able to handle several 100K books without problems.
Some of the issues are discussed here:
https://www.mobileread.com/forums/sho....php?p=2244745