The lid of one of my tanks squeeks when opened and the corys always get excited thinking it's food time.
Generally, corys are slow eaters and you can't expect them to eat the food in 3 minutes. I would be actually worried if all the food is gone in 3 minutes in a tank with corys. But I agree algae wafers should not be the main diet for corys. They need other stuff too, including foods high in protein and stuff like frozen blood worms. They also may not like the food you put in. I still have a pack of tetra algae wafers that takes ages to get eaten, even the pleco isn't interested, only when all the rest is gone some fish may nibble on it but as you say, I can see it for hours hanging around, not sure what the issue with them is.
My tanks which contain mostly bottom feeders, including the corys get many kinds of food as I love buying if I see something interesting: Hikari wafers for bottom feeders, hikari algae wafers, hikari mini sinking pellets, NLS thera +, NLS community, another 2 types of NLS I can't recall(all are small sinking pellets 1mm or so), tetra flakes, tetra pro colour, tetra cichlid sticks,sera algae wafers, frozen blood worms, mosquito larvae, brine shrimp, etc....and vegetables and fruit from the fridge too.
That's at the moment but it changes in time obviously.