I isolated the problem to the 4-port Anker USB hub I was using. Startup happens without any delay if I remove the USB hub, even with both 2TB SSDs plugged (directly) into the MacBook Pro. (It's strange, as I haven't encountered any other problems with the USB hub - it has been in constant use for Time Machine and Carbon Copy Cloner backups.)I've been having frustrating startup delays with my stable production system (2015 MacBook Pro 15") running macOS Sierra, where there's been a long wait after pressing the power key before the apple appears and boot starts. To make a long story short, the solution is simple enough, if an extra hassle: Disconnecting my two 2TB backup drives (SSDs) from USB3 eliminates the delay.