The activation of the ult is related to the ordinary activation of attacks for a hero. So the ult isn't used as soon as the meter is filled. I guess if you are unlucky with this in a few battles in a row, it may seem as if a hero is slow or doesn't do anything, I guess.
Can you explain mechanism a bit further?
I noticed that some ults are definitely quicker to activate than others.
Malrath seems champion here: he is really quick to ult.
Quite good are garrik and nightshade. Many times I saw garrik lucky to throw his tickets just before death from stupid Taurus hammer or nightshade jumping out, just to die in the backline of enemy team
Tareth, Taurus, oldman (that one is only to have excuse) seems to struggle with activation.