Shield has nothing to do with swing speed.
But does the weight of a shield not count? o.O
If we were looking at the realism stuff, which people tend to do these days, although only half of the realism stuff (lucky us) into the game, than it is correct that you can swing a katana faster than a nordic sword or something.
It is a very light, very thin and very fast sword and not longer than many onehanded swords.
+ you swing it with two hands, giving you much more power