"Every 20 minutes" for an S-bahn is totally fine! Especially compared to cities like New York, where every 20 minutes for some U-bahn equivalents would be considered good.
Every 20 minutes on one of the main lines is not fine. Even if it were fine, it's only fine if you can actually rely on the schedule, but not if you need to take into account a high chance that it's '1 hour wait and then three trains in direct succession.' Slow, but reliable service is ok. Slow and unreliable is not.