I have a MacBook Pro (Retina, 13-inch, Early 2015) on the latest version of macOS Sierra (10.12.3 at the time of writing). Whenever I hold the Option key after turning on my Mac, it takes a long time (around 30 seconds) for Startup Manager to appear. When it does, it is extremely laggy. Cursor movements and arrow key presses take 10-15 seconds to register.
I have no external devices connected; I am using the trackpad and keyboard built into the laptop. I eventually plan to connect a USB drive to boot from, but I cannot get Startup Manager to work correctly even when only Macintosh HD is available.
I have already tried resetting the SMC and PRAM multiple times, which does not help. Also, I tried installing the rEFInd boot manager to see if it would work better. However, rEFInd is just as laggy as the default Startup Manager.
The only other person online to have the same problem posted this question: OSX Startup Manager (Boot drive selection screen) slow/un-responsive in Macbook Pro (early 2015)
Unlike that person, however, running
diskutil verifyvolume on my
EFI volume comes back with no errors. Likewise,
diskutil repairvolume did not solve my problem. I have no idea what to try next.