Use of すごい as an adverb
While technically すごい is an い adjective and its adverbial form would conjugate as すごく, in modern Japanese most people would often just use すごい itself as an adverb as slang. It’s not even uncommon to hear this expression on TV (usually paired with 美味しい) even from older generations, to the point where I wouldn’t even consider it slang anymore and just plain normal Japanese.
Some native speakers even say it might feel wrong or weird to say すごく in this context, as per the exchange below:
Or even consider the すごく variant to be non-genuine and have an unintended double-meaning:
Since it comes across as less spontaneous and more calculated.