The more Dreads you wear, the heavier of course.
But how heavy Synthetic Dreads feel also depends on your scalp. If you have a sensitive scalp, you will suffer from a heavier weight faster than someone who does not have a sensitive scalp.
When you start wearing Synthetic Dreads for the first time, you will react more to the weight in the beginning than if you have been wearing Dreads for years, you are then completely used to the weight.
As I tell you in my video, I've been wearing dreads for four years now and the weight doesn't bother me anymore. I also wear more Dreads than we recommend because I like a lot of volume, but I'm used to them and my scalp isn't sensitive either.
And yes, Synthetic Dreads do get very heavy when they get wet, when you wash them for example. Then they are just as heavy, but luckily the Dreads dry very quickly.
Everyone experiences the weight of Synthetic Dreads differently, you really have to experience personally what weight you can carry and what you find comfortable.