Is this on Windows? I've experienced the same for the past few releases in Windows. For some reason that clone step is ridiculously slow.
I have heard it is to do with how windows antivirus works. Since the registry is like 10,000 seperate files. It chokes on them.
I have heard there is an upcoming feature to allow the package manager to work with the registry being kept inside a tarball, which is specifically being added to deal with this