MacBook boots up with less than one frame per second This is what booting my MacBook looks like: Macbook booting very slowly (youtube) 

I tried a PRAM reset, which was successful (it reseted the speaker volume and now plays the startup sound, which it didn't before), but that didn't fix the issue.

This is an early 2013 15'' Retina Mac, and I'm running El Capitan.

I really need this device, so any help would be appreciated!
Try also the SMC reset, doing it 2 or 3 times does not hurt:
https://support.apple.com/en-us/HT201295