The "speed limit" actually isn't speed at all. It is the ratio between time and distance elapsed from the last action. The game servers enforce a minimum time delay between successive actions based on how far apart they are. For example, if you hack two portals that are 1 km apart, you must wait about a minute. Normally, this isn't a problem because it will take at least a minute to travel between those two portals. But if you're on a highway or a GO train that can cover the distance in half the time, you'll notice you can hack the first portal, but not the second (you will get "hack acquired no items").
It isn't technically your speed that matters, because the game does not record that. You can hack a portal even if you are going 140 km/h past it, as long as you've waited long enough since your previous action. Conversely, you might not be able to hack a portal even though you are standing still because not enough time has passed.
This applies to hacking, attacking, deploying, recharging, dropping items, and picking up items. You can recycle items at any time because that does not have a location associated with it.