If Bitcoin price pumps from $1300 to $3000 in few hours nobody would make profit from it, because the exchanges wouldn't allow you to sell your Bitcoins and make the fast profit, it would bring a big loss for exchange sites.
Do you agree with this?
I do not.
Exchanges exist to provide their services to people by allowing them to exchange alts to bitcoin, bitcoin to alts, fiat to bitcoin/alts, and alts(if applicable or alts to bitcoin)/bitcoin to fiat. They do not choose the time when can their costumer be able to use their services, instead, the costumer chooses the time when they will use the services offered. If an Exchanger declines the costumer in using theis services such as selling their btc to fiat just because the price pumped hard, it would ruin their image and users may shift to other exchange that offers much better services and not only up to the profit.