Even a trained cashier cannot scan as fast as a trained cashier on these systems; they're slow by design. I got reasonably fast (but not cashier fast) on Safeway's and hit a wall: I kept running into false positive "unidentified item in bagging area", followed by clerk overrides. I eventually figured out that you can't place the item into the bagging area until the computer has processed it — there's a delay between the "beep" of the barcode scanner recognizing a barcode and the computer adding the item to the tab & then announcing the purchase, and you cannot hit the scale prior to that or it gets out of sync with you.

Also the only place truly training cashiers, AFAICT, is Aldi's.