Mock doesn't have COW (copy on write), cowbuilder by contrast creates a base chroot, then use copy on write to enrich it per build. This speeds-up things considerably.

Also, cowbuilder runs fine with several instance running at the same time, mock is a bit more sketchy in that regard. Basically, you can only build one package at a time. Typically, it's a bit of an issue for my personal scripts: https://github.com/kakwa/amkecpak, in them I can basically do make rpm -j 42 to build 42 packages at the same time, but with mock/mockchain, the chroot creation has some concurrency issues.

In fairness, I used the rather old mock version packaged by Debian, I've not looked at 1.4, it seems that some of these issues are addressed now.