Boot loop after flashing Android 6.0 Marshmallow on my Nexus 5

I had Android Lollipop installed on my Nexus 5 (Was Rooted, had TWRP and MultiROM installed) and wanted to simply start fresh with new clean install of Marshmallow.

So I followed the instructions (step 1 to 6): https://developers.google.com/android/nexus/images?hl=en

After running the flash-all script the device rebooted I see the Marshmallow boot animation but it has been running for 45 minutes. Is this normal?

Is there any suggestions beside reflashing because I've tried 3 times with the same results (45 minutes is the longest I've waited).

Many thanks for your time and help.