I've heard estimates of around 60mph. I dont know if that figure is straight flight, or in a dive. Either way, it's fantastically fast. All I know for sure is that, most of the time, they are faster than my net.
the 60 mph could be streight flight because dragonflys don't have that much weight to help themselvs go faster in a dive. and birds go faster because they close their wings, this is something a dragonfly can't do so i'd think this is streight flight.
Any falling object picks up considerable speed. If the falling object has a way of accelerating that decent (as a dragonfly would), it's downward speed can potentially be quite a bit faster than straight flight. Remember, it wouldnt just be falling towards the ground, it would be propelling its self as well.