I've never experienced a non-performant cross-platform UI.
Well, to be honest I've heard of folks complaining about Electron apps, but are easy enough to avoid. Oh yeah and Swing was a bit sluggish… in the 90s.
Allow me to somewhat quippishly rephrase your original post: "I've never heard of badly performing cross-platform apps, except for the ones written in what's currently the most popular cross-platform toolkit and the ones written in one of the most popular cross-platform toolkits a generation ago."