When you arrive at the main entrance of the Pantheon, you'll notice three separate queues:
The first queue is for visitors purchasing tickets on-site with cash. Wait times here can range from 40 minutes to 1.5 hours, especially during peak tourist seasons (summer months).
The middle queue, for those with pre-booked tickets, is the fastest-moving and allows entry at your scheduled time if you arrive 10-15 minutes early. If you purchase skip-the-line tickets in advance, this is where you'll find yourself.
The third queue is also for those booking last-minute tickets, but with card payment. While potentially faster than the cash queue, it can still involve a wait, particularly during peak times.