Not often enough! There are many barriers to bystanders starting CPR, including fear of "messing up", family members being too upset to start CPR despite receiving instructions from dispatchers, and a lack of adequate education in hands-only CPR, stopping people from helping because they think they have to do mouth-to-mouth.
The most positive outcomes have always come from when CPR and an AED are applied right away. Cases where a lay person starts CPR have always tended to have better outcomes and lead to the pre-hospital return of spontaneous respiration. I have also seen similar results in patients who have gone into cardiac arrest in front of me, having both basic and advanced life support already caring for the patient also leads to better outcomes.
You are performing a heroic act that can lead to saving someone's life, there is nothing you can do to make the patient's condition worse but by doing something, you can increase the chances of their condition getting better.
Paramedics bring the hospital to the patient instead of advanced care being deferred until the patient gets to the hospital. Paramedics provide advanced airway control which leads to efficient ventilations. Paramedics also carry the medicine necessary to stabilize a patient and or treat the underlying cause of the patient's cardiac arrest. Finally, paramedics can do heart monitoring, which allows the patient's electrical activity in their heart to be treated appropriately.
Not starting! Other than that, hand placement is key to success and I've seen some pretty inaccurate hand placement during CPR in my career.
This is something that most people don't see in their entire lifetime so when it occurs it can be unpleasant and surprising causing your adrenaline to kick in your "fight or flight" instincts. The best thing you can do is get your nerves under control quickly. Take a series of deep breaths to steady yourself and then jump into action, your timely intervention can lead to better outcomes than waiting for the ambulance to show up.