I can imagine a production wherein this is not an aside – but spoken to a co-conspirator. He could say it to Laertes, for example. Or to Osric – if Osric is in on it. Or just some minion he’s brought into his confidence. If spoken to someone, the line takes on an urgency that the fact stated as an aisde lacks. If he says it to someone, he may still hope that someone can do something about it. And then it is too late. There’s a sort of implied hope in this first sentence of the line if spoken to someone. Not hope, exactly – just, it’s not too late yet. And then it is.
The right actor could probably imbue the line with this even without saying it to someone else. He could be attempting to tell himself to do something, you fool. And then – welp- she’s drunk it, it is too late.
It’s kind of a funny line. We all know it’s the poisoned cup. Laertes knows, too. But perhaps Shakespeare is just making sure that anyone who slept through or was talking during these bits before now gets that Gertrude is about to drink some deadly poison.