I don't think it's the ECU here. This may be a PIA to try but maybe take the starter motor out of the equation.
Disconnect it from bike system (electrically) and direct connect it to another battery sitting next to the bike that is fully charged (obviously) with different cables, don't use garbage jumper cables.
Crank it like normal and see if it fires up first shot and quickly. This would probably make targeting the issue much easier.
Still slow I'd look at the starter motor if it's faster and sparks up wiring relays or battery, then it's parts swapping time