Short version:
please remove it

Long version:
I have never really counted, so I'd say that there are more than one people asking for open world content to be "more dangerous". Dangerous as in there are open world content that can kill you even if you're iLevel 70. I can symphatize with that, but Heavy is NOT being more dangerous

A Wild Heavy Appears
Heavy is mostly inflicted by high level monsters (the ones near Primal grounds) or the inconspicuous ones (such as the Overgrown Offerings in South Shroud). The problem with Heavy is that it's simply annoying, because:
  1. Monster-inflicted Heavy is not subject to Diminishing Returns. This means that every time Heavy procs, you'll be slowed for 20 more seconds
  2. Most, if not all Heavy inflicting attacks are ranged
  3. The monsters can chase you for quite a distance. Especially in flat areas such as Sylphlands

Not Really Useful
The monsters in question generally can't kill you, and when they can Heavy is generally the least of your problems. In an open world, people just want to move from point A to point B, and not interested in killing monsters. When they DO want to kill the monsters, they don't need Heavy as an incentive to stay

Sure, if you're undergeared, Heavy is a difference between life and death, but that's all its use. Once you are better geared and have better defense/HP, Heavy is just another annoying thing in the open world

Just Please, Remove It
I don't know the purpose of Heavy, but if the purpose is "to make the world more dangerous". It simply hasn't accomplished it, and frankly will never work by itself. If you want to make the world more dangerous, please also give a way to counter it. And no, walking together with a healer everywhere is not an option. Would you rather make the DF even drier?