Again, that’s better served by other means. Cardiac arrest survivors typically have already been shocked before ambulance arrival. Even the best ambulance response times in the world are typically well outside the acceptable time range.

You chose a good example though. There is no more time critical situation, and it’s also a great example of why this isn’t really a good use of money.

Edit: to expand upon this, prehospital research is unbelievably difficult to do and typically gives unpredictable results, but it's very clear that the factors that allow people to survive cardiac arrest specifically are recognition, CPR, and pre-ambulance defibrillation. Ambulance-specific interventions are only weakly supported. Trials vary, but it's usually pretty uncommon for ambulances to arrive within an acceptable time range from the moment of collapse - bearing in mind that the patient doesn't get shocked until the ambulance has arrived, the crew has grabbed their kits from the vehicle, walked to the patient, identified the arrest, attached their monitoring, *then* hit the shock button. Ambulance arrival at five minutes means, at best, a shock at maybe seven.

If you're going to spend millions to reduce the time delay to arrival, your money is probably better spent providing really high quality community CPR education, public access defibrillators, and staffing more ambulance crews. You need to accept that most of this will never be used, but it's available for when it's needed.