OSX Startup Manager (Boot drive selection screen) slow/un-responsive in Macbook Pro (early 2015)

According to Apple, the EFI partition is not used to boot OS X. The interesting think about the damaged Firmware.scap file, is that evidently the file is copied from the /usr/standalone/i386 folder to the /volumes/efi/EFI/APPLE/EXTENSIONS folder at boot time. See web site: /Volumes/EFI - Firmware.scap & MBA41_0077_B12_LOCKED.scap. If this is true, then you should not have to worry about replacing the damaged Firmware.scap file.

I suppose the next step would be to try and fix the EFI partition. If you wish, you could first backup this partition to a file. The file would be an exact image of the entire partition. To backup the entire EFI partition to the file named efi.binary, use the OS X command below. The partition should not be mounted, before entering the command.

sudo dd if=/dev/disk0s1 of=efi.binary bs=409600

If something goes wrong, you can at least restore the EFI partition back to its original (faulty) condition. To restore the entire EFI partition from the file efi.binary, use the OS X command below. The partition should not be mounted, before entering the command.

sudo dd if=efi.binary of=/dev/disk0s1 bs=409600

The OS X command to repair the partition is given below.

diskutil repairvolume disk0s1

An alternative would be to use Windows to run chkdsk on the EFI partition. From a Administrator Command Prompt window, you could enter the following commands.

mountvol s: /s
chkdsk /f s:
mountvol s: /d

More information, about accessing the EFI partition from Windows, can be found at the web site: How do I mount the EFI partition on Windows 8.1 so that it is readable and writeable?