The delay is intentionally. If memory serves me correctly this was so that users who "wander" with their mouse weren't getting inundated with fly-outs. The small delay offers that relief.

The delay is either too long, or the menu should respond immediately to clicks. As a user, it feels like I'm wasting time waiting on the menu to open. Also, the horizontal navigation has always opened immediately, so I would think that users are accustomed to that behavior. Is this delay a result of feedback from UX trials? I would be curious how many users out of the group commented on this aspect of the menu.

+1 to comment 3, if menu opens immediately after click, it would be a win :-)

So, I did some research, and patternfly doesn't allow overriding this delay, though there's an issue/feature request to add it [1]. I went ahead and submitted a PR to patternfly [2] to implement the feature, since the conversation on the feature request seemed stale.

[1] https://github.com/patternfly/patternfly/issues/744
[2] https://github.com/patternfly/patternfly/pull/1073