Erm...
I'm no legal expert, and maybe one can correct me if I'm wrong, but the hearing today is in realtion to an extradiation request, not the crimes themselves.
In the cases you mention, "serial rapists, mass murders etc" the authorituies need time to collate evidence for and a against i.e make sure they have enough information to put the person in prison (similarly the defence needs time to prove innocence if applicable).
I the case of Mr Assange, they simply need to determine that he is the man that the extradition is for (he handed himself in so we can assume that is covered) and that he will be given a fair trial if he is handed over.
Again, I'm no legal expert, but this is why I believe the case can be heard so quickly.