The compute differential between an H100 and a 4090 is not huge. The main single GPU benefits are larger memory (and thus memory bandwidth) and native fp8. But these matter less for diffusion models.
Its why fal, replicate, pretty much all big diffusion api providers use h100
tldr; 4090 is max 3.51 it/s even with all the current optimizations. h100 is 11.5it/s with all optimizations, and even without its 6.1 it/s
Paper discussing here is using 4 bit compute, which is 4x on 4090 in comparison with bf16 compute, while h100 doesn't have this at all (i.e. best you can get is 2x compute with fp8). So this paper will even out difference between those two to some extent. If to judge by theoretical numbers - H100 has 1979 TFLOPs fp8 compute, and 4090 has 1321 TOPS. Which puts it around ~65% of performance. Given the price of it ~$2K compared to H100s ~$30K this seems like a very good deal.
But again, no 4090 in DCs.