I just engineered a situation in my current game to attack an upgraded argonev with an orkulus. I still had to jump my fleet in to protect the orkulus while it was upgrading it's defenses. Building it and upgrading it took what seemed an eternity, more like 6-8 minutes, i guess? Then it took just as long to kill the argonev, even though it was upgraded to only ~12k hull. The only thing in the orkulus corner is that once the sb was actually dead, it still had all of its hull and ~5% of its shield.
That seems like a very long time to tie up my fleet, since the orkulus on it's own I think is still vulnerable to large fleets.