After updating to Sourcetree 4.2.10 (269) for macOS opening some repositories is quite slow. Reverting back to 4.2.9 makes them open quickly again.
Looking a the debug task monitor the problem seems to be related to the 'list repo config files' step that runs a script called 'list-repo-config-files.sh' (this appears to be new in version 4.2.10).
If the repository has lots of directories then this script is quite slow.
I believe it might be missing a '-maxdepth 0' in the find command like this:
which would make it only search for a ./git/config or /config file at the root of the passed in directory (rather than recursively in in all directories).