Although not supersonic it
was required to achieve M0.9 at low level, this, combined with a max t/o weight of about 160000lbs may go some way to explaining it. The nature of a flying boats landing medium does result in a built in headwind.
Turbojets were still pretty gutless in the early '50's - the original design was going to use turbo-ramjets, but after development issues these were replaced by afterburning Allison j71s in the prototype and pre production aircraft. As far as I know, the production series had non afterburning J75s that were rated at about 17000lbs.
This seems to be the most authoritative detail on the web.