I'm sure the owners of your store would disagree. And if they wouldn't, then they're really harming their bottom lines. If it weren't for customers you (and others) wouldn't be employed.
I did retail. Currentlly I work with an automated voice response system for a large bank. Believe me when I say that I can understand that certain customers are more work than others and don't 'get' the system or the process. But I'm fully aware that w/o customers, I wouldn't have my job. The perceived fault of the customers not understanding something is not THEIR fault; it's mine. It's not their problem; it's mine. They are not supposed to be the experts, I am.
20 seconds on the customer" -- WOW, that speaks volumes.
I'm curious, have you said this to the owner?
This is why people are moving towards the internet for their shopping.