Reinstall macOS on Ubuntu MBP

Give this a try. If you can get to single user mode you can remove the file that OS X checks for during boot to determine if the Mac has already been setup. If it doesn't find the file then it boots to the setup assistant, which allows you to create a new admin user.

Try resetting PRAM:

1. Shutdown the Mac
2. While holding ⌘+option+P+R, power on the Mac
3. Continue holding all four keys until the Mac has chimed (restarted) twice, then release the keys and allow it to boot normally

If that fails, or you still can't boot to single user mode, give this a try. Resetting NVRAM:

1. While holding ⌘+option+O+F, power on your Mac, after a few seconds you should be greeted with the Open Firmware screen.
2. Type and click return: reset-nvram
3. Type and click return: `reset-all'

Hopefully by now you can boot to single user mode.

1. Boot to single user mode (⌘+S while powering on)
2. Type and hit return: /sbin/fsck -fy
3. Type and hit return: /sbin/mount -uw /
4. Type and hit return: rm /var/db/.AppleSetupDone
5. Type and hit return: reboot

This should boot your Mac to the setup assistant. From here you can create a new user account, which will automatically be designated as an administrator account.

Hope that helps.

Same issue here, noticed it in FileVault, but then realized it is there even on Boot selection section or even Diagnosis section. Disabling FileVault won't solve the issue, it will just cover it up.

I wasted a lot of time with software sides of things, reinstalling OS and all. they all didn't help. It is a Hardware issue.

I realized I have this issue once I started using SD Card reader for storing my files and extending my 128gb MBP. If SD Cards goes in, even for 1 second, and then take it out, boot is going to be slow until I do a NVRAM reset. Apple care didn't find any issue first time but second time they admit there is a hardware issue in my Macbook and they replaced it. I will report back issue returned.

You need to bring it to Apple care, and describe the issue in detail. I had to print a A4 paper with a detailed description (told them issue happen when put in a sd card and then take it out) and screenshot of diagnosis page showing SD Card error in diagnosis boot page (because for some reasons, it was randomly showing that error and apparently when I give my Macbook for first repair, it passed test 100% successfully) Reception who receive my Macbook, she didn't really reflect things I told to her to technician. She even typed issue of my macbook as "Booth up slow", you get the idea.

Yeah, you need to bring it to apple care for hardware fix and bring enough documents to prove them it is hardware issue.