I was rsync-ing a directory to remote host using an old version of rsync, so it had to count all the files on my local machine before it could start copying (newer versions of rsync start copying before reading all the files). However this was a large directory tree, with thousands and thousands of files, and it was taking rsync a long time to count. There are tools like ncdu to show how much disk space each folder is using. However in this case I want to reduce the number of files, regardless of their size, so ncdu is useless to me.
This bash snippet will show how many files & directories are in each folder, with the most 'populous' directories at the bottom.
find | cut -d/ -f2 | uniq -c | sort -n