Please describe the problem:
Let $src and $dst be 2 directories needed to be compared and merged. When $src on ly have 2 files which have been modified and $dst have 1000 files. I found the comparison process is extremely slow. For example, $dst/device/ is existing and $src/device is not existing.  All sub-dir and files of $dst/device don't need to be compared!

Steps to reproduce:
1. Two directories
2. One contains 2 files
3. The other contains 1000 files and many directories.


Actual results:
Work properly but extremely slow.

Expected results:
Be quick, don't waste time.

Does this happen every time?
Yes.

Other information: