As a general rule, if you're getting a service for free, you're the product.
I don't know much about the downsides of free in the context of VPNs, as I didn't really get in to the technical ins and outs. But when we're talking security and privacy I think the cost of supporting something good and sustainable is well worth it.
I've heard port forwarding is helpful for speed, and that might be a paid only thing, but to be honest I'm consistently surprised by how fast things go for me just using mullvad.
Another thing you may wish to consider: the Proton CEO has praised Trump which is a huge red flag to me in the privacy space.
It works for me... I think it's something like it goes faster with port forwarding but to be honest I don't know the details.