What sparkfun product number are you using? Is it an actual sparkfun product or is it a clone?I had pruchased and used 2 pro mini 5s
Bootloaders *do* cause a delay at startup because that's when they are running. They are looking to see if you're uploading new code during that delay.It appears the bootloader had caused this delay.
As long as a bootloader is on your device, there will be a delay. If you want to continue using the device with a bootloader you will need to add a delay to your I2C device scan so that it happens after your code is running on your device. You could load your sketch with a programmer and overwrite the bootloader to eliminate any delay but then you can only program via a programmer.This delay caused the Pro Mini to miss a scan fo I2C devices and subsequently would not be detected in the system.