I haven't compared my Kojaks to any other tyres on the same bike - so I'm limited to what I can conclude, but....
Handling an unfitted Kojak with my fingers, it feels reasonably flexible, and the sidewalls are skinny, so the rolling resistance shouldn't be terribly high. Of course it's not like a real racing tyre, such as Vittoria Open Corsa, but it's less stiff than some alternatives.
The size might be a factor, however. A larger tyre, if not over-inflated, might be faster on bumpy roads in the real world, even if the material is stiffer. Remember, though, that a larger rear tyre raises the gearing.