I don't know about less than 1 second but I just picked up an RTX 3090 Ti now that they're basically half off at Best Buy and it's definitely fast enough for interactively playing with prompts (single digit number of seconds).

Probably overkill and could get away with something like a 3060 or so, but the 24 GB of VRAM come in handy if you want to generate larger images. I pushed it as high as 17 GB on some recent runs.