Activating Hardcore AI, downloads a more advanced computer model; "brain" if you like. This download is only done once.
Using this larger model (brain) takes more computer power, which corresponds to taking longer time. It's a cost, but the benefit is the production of some more "creative" work from the Mr. Dalle AI painter :)
I think that if you compare the results from Hardcore mode on/off, you'll notice an artistic difference in the pictures.