If it helps at all, this is a mini timeline I did a while back. Three mins between EMTs arriving and CPR being started. (UK) paramedic that I spoke to found this to be a surprising delay.
20:27.45 EMT Smith checks for carotid pulse and checks pupils. All three police officers are still holding GF down
20:28.40 EMTs have stretcher ready. All three police officers are still on GF
20:27.45 police officers get off and GF is loaded onto stretcher still cuffed
20:29.30 GF is in the ambulance.
20:29.42 coppers belting him to the stretcher. EMT removes GF’s cuffs and vest
20:30.43 Smith checking again for a pulse. Still no chest compressions
20:30.47 Officer Lane begins chest compressions at EMT’s direction