Based on the demo, that's... incredibly fast. Literally generating images faster than I can type a prompt. They've clearly got a set seed, so they're probably caching request, but even with prompts that they couldn't possibly have cached it's within a second or so.
This isn't cached. I'm running it locally on a 4060 TI 16 GB and it's just as fast. Image gens in .6-.8 seconds. Each word or character I type is a new image gen and it's INSTANT.