When rescuers are worried about opening the airway and making an adequate seal, plus the "ick" factor and possibly digging a CPR mask out of a purse or briefcase, the delay can be significant. All that extra time is getting in the way of real help: Chest compressions.
In its summary of the changes, the American Heart Association explained it this way:
In the A-B-C sequence chest compressions are often delayed while the responder opens the airway to give mouth-to-mouth breaths or retrieves a barrier device or other ventilation equipment. By changing the sequence to C-A-B, chest compressions will be initiated sooner and ventilation only minimally delayed until completion of the first cycle of chest compressions (30 compressions should be accomplished in approximately 18 seconds).
So as you can see, by starting chest compressions first, the victim only has to hold his breath an extra 18 seconds while blood gets flowing again. In my opinion, that's a good trade.
Field JM, Hazinski MF, Sayre MR, Chameides L, Schexnayder SM, Hemphill R, Samson RA, Kattwinkel J, Berg RA, Bhanji F, Cave DM, Jauch EC, Kudenchuk PJ, Neumar RW, Peberdy MA, Perlman JM, Sinz E, Travers AH, Berg MD, Billi JE, Eigel B, Hickey RW, Kleinman ME, Link MS, Morrison LJ, O’Connor RE, Shuster M, Callaway CW, Cucchiara B, Ferguson JD, Rea TD, Vanden Hoek TL. "Part 1: executive summary: 2010 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care." Circulation. 2010;122(suppl 3):S640–S656.