My MacBook Pro 2017 was lightning fast at startup, then I installed macOS High Sierra on an external disk and tried to boot my mac from that disk. Everything fine.
Now whenever I boot my Mac normally (without even having the disk connected) it gets stuck at the grey screen before the Apple logo for a lot of time, like 5–6 seconds, something that never happened before.
It looks like it is spending time deciding which disk to boot, even though there is only the Macintosh HD. Is there a way to fix this?