It's crazy. Are they doing any precomputing as you type, I wonder if you paste a block of text is it the same speed.

I pasted and instantly hit enter on this prompt: "I generated a filter set using REW v5.31.3 using real-world sweep tone measurements from the room I'm listening in . How can I use it as my MacOS output equalizer so that my spotify music is adjusted for this room and speakers"

and it gave a very reasonable answer in non-perceptible time.

No, it really does take ~0.03s to generate the answer. Try your browser's developer tools and watch the requests.