It all depends, releasing big instant may merely be impossible because of the key word there "big", because they may have to make some information and run a check before they can confirm for this, also, if we see it as something that we need to to cash out instantly, then we must ensure that the conditions for withdrawal have been meant and then we may proceed for the next step of action, they just want to ensure that they don't send the amount of money to a wrong hand and also make the necessary verifications from the user end, so that they cant afford any form of mistake at the process, since large amount of money is involved.
I would say up until now, only few casinos can do it instantly without a doubt. As long as there are no problems with your KYC docs and you haven't violated any of their terms. But if some casinos will take a pause, for me, it is understandable as we are talking big amount of money. And it can really have impact on their vault especially for small casinos.