XF doesn't support that level of cooldown granularity so we can't just because of that, however...
The current thread-creation-ratelimit is, I think, good enough?
We had to deal with a significant wave of spammers lately and this was a way we had to make their life harder.
So we adjusted it to the higher end of what we consider reasonable: 15 minutes between new threads.
And I fail to see when one would really need more than that tbh. Sure, in extreme cases that might be warranted, and then it's a slight inconvenience, but how often does that happen really? And isn't it fine to just make a coffee between threads in such rare cases?