It is a bit unclear if you intend to split the libraries so each library only contain one format, or if you intend to split using some other criteria, and sort on formats in the separate libraries. Here I assume the latter.
To have books sorted by format is quite expensive. It typically means that when opening or switching library, Calibre starts checking every book in every book folder to see what formats there are for each book. This can be VERY time consuming. I don't think it matters much if the library is split or not. It will be faster to open a smaller library, but most likely you will switch often, so you still end up just as annoyed.
A better approach might be to convert books so you don't need to keep track of the formats. The main problem is PDFs. Typically it is hard to automatically convert a PDF to any other format. I have ended up converting everything to ePub, or keeping it as PDF. Since my main ereader, Mantano, handles both ePubs and PDFs, this works out great.
Another way to speed up sorting of books by format is to change how the sorting is done. The slow default way is to sort using a custom column that holds a value that is created by checking each book. This is slow but safe.
A faster way is to change the sort to use information already stored in the database, about the formats that are available. This is much faster, but not as safe. There is no check to see if every book actually still is there. If there was some problem with the actual books stored in the library, you may not discover that until it is too late to do anything about it.
You can read more about this here: