Boot loop after flashing Android 6.0 Marshmallow on my Nexus 5
OK the problem was that I used the wrong version of fastboot. It was already installed on my Mac so when I was running the flash-all script it was actually running the system version.

So I modified the script to make sure the path to fastboot was actually pointing to the good version.

And voilà, it took about 7 minutes after reboot.