Sometimes there are also unimplemented/broken features on Linux which people don’t notice and save frames. Legit performance improvements over Windows do happen (especially on memory and cpu-limited systems) but I’d be skeptical of any particularly huge ones.
Unity started with pretty awful performance (much like GNOME 3) and coincided with some infamous decisions on the part of Canonical, namely that whole business with the Amazon integration, so it’s permanently tainted in the minds of many. It also meant that the largest distro in town was suddenly using a desktop that was much less inviting to newcomers than the familiar GNOME 2.
I’m glad it’s being kept alive as it does have a unique vibe to it, but I always found the workflow a bit awkward and much prefer GNOME for something modern and xfce or MATE for when I want something traditional.