I recall reading about it somewhere. The reason for the couple seconds of pause before the sketch runs is due to the boot loader waiting for commands for flashing a new code. You may want to do some research but it should be possible to set to run immediately but It may disable reprogramming without resetting fuses in the future.
You might want to ask in one of Adafruit's forums, the bootloader on their itsybitsy M0 is considerably different from the optiboot used on most arduinos, so I'm not sure what would be involved. On most arduinos, when you first power on the board it automatically skips over the delay in the bootloader and goes immediately to the sketch, and you can completely eliminate the bootloader by loading the sketch using an ISP programmer.