Yes it's super slow, but many of the criticisms are missing the point. This enables browsing over SSH when you have low bandwidth. The heavy internetting is done on the
remote machine (like a cloud VM for example) using blazing fast data center internet. A personal example may explain why I love this.
For several years I lived in rural Alaska where the fastest internet one could buy was $120 a month (I think) and a blazing 512 Kbps. I was a developer (working remotely) who's shop had adopted Docker, and it literally took more than 24 hours sometimes to download a docker image. By necessity I switched to having my whole development environment on a cloud VM. The Cloud VM had gigabit connection so docker downloads were blazing fast. All I needed to send across the wire was a tiny bit of text. Mosh was an absolute life saver by the way. I once flew from Anchorage to Salt Lake and had the same mosh session pick up like nothing had happened thanks to roaming abilities.
Browsing heavy (i.e. modern) websites was often very difficult too. With high latency and a lot of heavy Javascript sites requiring 10 MB or more, it was a nightmare. I occasionally went up to Eagle Alaska, where internet was even worse. The nearest cell tower was a 4 hour drive away, and the only internet was at the "library" or a crappy satellite link (that far north satellites get less useful). A tool like Browsh is a life line to people in situations like that.
In related news, when people talk about the merits of developing with just Vim vs. an IDE, I also recount the same story.