Date: Monday, February 1st, 1999, 00:00
In TIL article 24763, Apple explains why there might be a slight delay and a flashing question mark after installing the iMac firmware update.
The firmware update resets certain settings, such as the designated startup disk. This causes the iMac to search for a startup device, which in turn causes the slight delay you describe.
After the firmware update has successfully completed, restart the iMac. Once it has fully restarted, use the Startup Disk control panel to set the internal hard drive as the startup disk. This action will eliminate the blinking question mark and the delay.