I am also noticing a Loading 'Ubuntu' message while the Grub background image is still on screen. It appears before the sparse file error message (which we get when using btrfs). I take this Loading Ubuntu message as an indication that Grub has located its grub.cfg file and has read it. I think that the loading Grub message would stay on screen longer if Grub could not locate grub.cfg. I wonder what message we would get if Grub could not find the partition the kernel was on or the kernel. I am not about to deliberately break a working install to find out. Perhaps this is all to do with Grub being more informative when things go wrong other than a Grub Rescue message.