Without port forwarding you can only connect to those who do have it set up. Doing so yourself allows you to be a better citizen of the internet and share with people who don't know what it is.
(Caveat: I am one of those people who don't understand it and am just parroting what was explained to me when I asked about this)
I think it's both? I'm assuming you get fewer seeds you can connect to as well.
Either way, seeding issues are leeching issues, in the grand scheme of things.
Edit: I only stress the seed side because I've found I get things plenty fast even with Mullvad, so not explaining the issue and just talking about down speed can make it seem unnecessary, when it is in fact critical to the health of the community.