It's an item not a perk.
Perks have no drawback (:
But "must have" items which don't break / consume resource are lame.
Grenades and healing stuff consume resource, that's why it has no drawback.
Running with a backpack with 50kg of stuff is hard, that's why 5% less running speed is also pretty realistic.
Running in some heavy combat armor or certainly PA is hard but I do not see a speed reduction. I do not think realism should be brought into the matter. This is a Fallout Online game not Arma. Hell on the combat armor sprite it is clear a backpack is already there. So if we were to be realistic about the matter, combat armor should already have a speed reduction by default.
Armor and weapons are not also resources? They are pretty much consumed from use or when you die and are looted. They are as much a resource as any other, but with much more longevity and the potential for permanent existance due to repairing but the act of repairing also consumes resources.
Though getting a bit sidetracked; utility is seems to have an obvious place as more of a way of carrying around a tactical tool to help in certain situations based on your abilities. Medics/doctors have first aid kits and defibs, etc. People who need to carry an explosive for buildings, cannot reload at the time, and such can carry some grenades and do better with some throwing skill. People who have low strength can carry a backpack. Not to mention, this doesn't help with combat, so it's not unbalanced and thus I don't see the logic in giving it a drawback except just for the sake of.
I think I agree with nunn however, that this item should be more of a luxury item, comparable to defibs in price perhaps. So it's for people who aren't simply running in to core naked with a backpack to grab as much phat lewt as they possibly can. I certainly wouldn't think it craftable with some brahmin hides or anything, which is a proper drawback in itself I think.