With some older NVIDIA graphics cards mostly from GTX 900 series and at least one APFS partion in your system you might see a screen like this right before Apple logo or boot picker.

The reason for this is some older version of our EFI firmware which conflicts with APFS boot. This doesn't affect boot process or boot picker functionality. Only boot can be delayed a few seconds until APFS initalization completes.

If it still bothers you might need to reimage your drive from APFS filesystem to HFS+ using some imaging software like e.g. SuperDuper. After that you can remove APFS partition which will resolve this problem right away.

If you see such screen on AMD cards please reset NVRAM by pressing and holding Option + Command + P + R keys (on Mac keyboard) or ALT + Windows logo + P + R (on Windows keyboard) until you hear loud Apple chime from the built-in speaker.