Again, you are mistaking magnitude for lack of speed. Two Huge creatures engaging in combat are going to be moving fast compared to each other, but when perceived by smaller creatures they will seem to move slow due to the sheer size of said creatures.
As big a target as Ulyaoth presents, it is unlikely Kain would be able to get within combat range to use the Reaver. Beyond simply using portals to keep Kain at a distance, he can summon barriers to block the vampire's path which would drain his energy if he were to attempt to get close to it.
You would have to prove this is the case this time. It still does not help your argument for speed, if their fastest speed seen is according to you from the point of view of a small entity then youll have to provide evidence for your opposite argument for them being fast. And your talking physical speed, I am talking the speed it takes for the thing to create portals, no physical movement was made, merely a flash of light within it. Too slow.
How will useing portals help him at all? he will not be able to react to Kains speed, nor will he be able to create portals quickly enough to stop Kain even if he could react to him so its hardly got a chance this jellyfish. Kain could open his own shop afer his most recent dispatch of giant sea life.