Currently, the hovercard delay is 150ms. I saw many comments that indicated that this delay will (or should) be made longer, but did not find anything from the developers indicating that they agree, and did not see any discussion regarding the correct delay for displaying the card. i created Bugzilla:66301 asking for the delay to be increased to 500ms.
if you have any opinion on the question what should be the right delay (even if this opinion is that 150ms is best), or (better yet) you can point to some "industry standards" or "best practice" or "UI guru" that has some guidelines regarding the best delay, please add it either here, to bugzilla, or both.
peace.