From the manpage:
Beginning with rsync 3.0.0, the recursive algorithm used is now an
incremental scan that uses much less memory than before and begins the
transfer after the scanning of the first few directories have been completed.
This incremental scan only affects our recursion algorithm, and does not
change a non-recursive transfer. It is also only possible when both ends of
the transfer are at least version 3.0.0.
Some options require rsync to know the full file list, so these
options disable the incremental recursion mode. These include:
Because of this, the default delete mode when you specify --delete is now
--delete-during when both ends of the connection are at least 3.0.0
--delete-during to request this improved deletion mode
explicitly). See also the
--delete-delay option that is a better choice than
If you're using any of those mentioned options or older versions of rsync you may be disabling the improved incremental recursion mode. Otherwise the only other option is to give rsync fewer files to consider.