Blank screen before booting on Macbook Pro w/ Retina Display

When i turn my Macbook Pro (early 2015) it hangs on this black screen, it never used to do this, and i also find that my boot screen (where you pick mac or windows) is extremely laggy or delayed, i recently unistalled windows and done a clean install to sierra (this didnt fix the problem) and also reset the PRAM with no luck either!

Any help is appreciated!