First responders are generally at a disadvantage from the get-go because some time has already passed between when a cardiac arrest happens and when they show up and begin CPR. Every second counts.

However, a large prospective observational study published in 2010 by the folks in Phoenix who were among the vanguard of advocating compression-only CPR observed that among adult cardiac arrest victims outside of a medical setting who had a layperson perform CPR (traditional and compression-only) survived to hospital discharge 7.8% and 13.3% of the time, respectively. Unfortunately, out of the 4,400 cardiac arrest victims they tracked over 5 years, 2,900 victims did not get any CPR at all (at least by lay bystanders).

We might presume that there could've been another 200-300 "saves" over that time if all of them had received CPR of some sort.

Edit: actually, on second thought, that's an overestimation. About 5% of victims who did not get any CPR survived to hospital discharge so more bystander CPR would not save that many. So maybe half of my original estimate that with compression-only CPR. Still, if that saves someone you love, even one more save is significant.