It seems to be about half a second now, but varies a little bit depending on where I am accessing it. Maybe it just displays as soon as the data is available?
I think it would be worth trying one second.
In fact, I was thinking it would be good to have a Toolbar Button to turn Tooltips on and off, as when I am working on TV Show tagging and organisation, the Tooltip just gets in the way no matter what, unless I make the effort to move the mouse cursor to the top on MC. Then I looked, and there is one: "Toggle Tooltips".
The trouble is, it doesn't work until you move to a different View and back again. Refreshing the View doesn't activate the change. If Tooltips are on and I toggle the settings using the button or the Option dialogue, Tooltips keep showing until I switch to another View and switch back to the View I was using. Same in reverse, activating Tooltips.
If that Toggle Tooltips button worked immediately, much of my frustration would be removed. Then a half second, or the current Tooltip delay, would be fine I think.