Debugging boot performance issues in grub / before kernel logging starts

My PC takes a long time booting. I have reason to believe it is caused by delays after grub, but before kernel logging starts (booting takes 30 seconds, but dmesg messages have time stamps between 0.00000 - 9.34223, details in [this post](https://unix.stackexchange.com/questions/499832/long-time-spent-between-grub2-menu-before-kernel-boot)) . Are there any ways to debug what's going on? In particular: * Is there a way to make grub itself more verbose, or keep logs? * Are there any other processes between grub and the kernel logging that could be taking time? How do I debug those? I believe the question is not specific to my setup. But just in case, I'm running Ubuntu 16.10 and grub(2) **EDIT**: Following a suggestion by @TooTea, I set the debug=all environment variables which produced a lot of messages a la script/script.c:50 malloc 0x7a9a2ca0 that should not be a problem. Then there is one block of messages that coincide with a 8 seconds delay delay: kern/dl.c:56 Detecting ext2... lib/relocator.c:1397 chunks = 0x7a7e0ae0 lib/relocator.c:434 trying to allocate in ...-... aligned ... size ... lib/relocator.c:1198 allocated: ...+... lib/relocator.c:1409 allocated .../... lib/relocator.c:1410 chunks = 0x7a7e0ae0