That would be fundamentally wrong. At least in the case of cash equity, as soon as a buyer matches a seller, the price of the instrument should be updated in r

That would be fundamentally wrong. At least in the case of cash equity, as soon as a buyer matches a seller, the price of the instrument should be updated in real time. Same applies to inserting orders in the order book, there should be zero delay, so that the exchange can halt to auction as soon as it detects out of the norm bids (or asks).