Performance issue on commit with many changed files

We have git repositories where the first commit (initial import) contains thousands of files.
With Sourcetree on Windows, selecting that commit virtually hangs the application (for minutes).
Same thing for another commit than the first one if it changes a lot of files.
Until version 1.6.8, this problem didn't exist, it appeared from 1.6.9 and is still present in 1.6.11, as I discovered by trying successive versions.
OSX versions do not have the problem either.