Moderator Notice
Moved out of recipes
I don't think you will find anyplace in the documentation that advocates adding 5K books at a whack.
If the books
did contain perfect metadata, doing a 1 or 2C at a time might be reasonable. Still you should probably take time to resolve any problems before processing the next batch.
If the books do NOT contain the metadata, Calibre throttles metadata lookup, being a good Net Citizen, as to not get banned from the nice folk that provide the metadata.