I studied statistics as a second major, and the "bank teller" problem is classic (Wrote solution in SPSS! lol)
If you feel that you are always in the slowest line, then check what's in their carts.
Is there a bagger, or does the teller have to stop ringing/scanning and do the bagging also?
Do people have their wallets ready, credit/debit card, or a ton of coupons and bags of change? ;)
There are a lot of factors and always the outcome for efficiency should be 1 line, n tellers, not n lines, n tellers.