Ven tan rápido como sea posible.

Does anyone understand why the subjunctive is used here?

“Come as quickly as is possible.”
No doubt, if you ask a number of people, “Just how fast is as fast as possible?” you’d receive many different opinions - it’s all very subjective. So, in this case the subjunctive is used because opinions in Spanish are expressed using the subjunctive mood.

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Thank you for the explanation, blogscot!