To combine the libraries select all the books and use the copy to library function --- and come back the next day
There are shortcuts, but they can easily become long detours when bad thing happen.
To optimize --
Depends what is slow ....
Some other things you can try (if you haven't already):
Upgrade to a recent version of calibre
Hide the tag browser & cover browser
Set your anti-virus to exclude files in your library folders
Set your anti-virus to exclude the calibre.exe and calibre-parallel.exe processes
Sorting on composite columns can slow it down a lot -- but it only happens once -- when you sort, or when you open the library. If you're talking about slow startup/library switching you may be doing a sort without realizing it.
Defragging the drive has helped some people.
Speed up slow searching by excluding the comments field (Preferences-Searching-Limit the searched metadata)