The performance issues are what prevented me from using SMIL. A project I was working on called for an SVG animation that we also had to move on scroll (don't ask...). The animation was choppy and definitely got the fans spinning.
Swapping that out for the equivalent CSS animation and everything was smooth as butter. It's obvious that SMIL performance is not as important to the browser makers than CSS animation performance.
I had the opposite experience a few years ago, where SMIL performed better than CSS animation but it likely varies depending on what is animated. Problem was only that certain browsers at the time (old Edge for ex.) didn't support it. Thankfully Chromium and FF continue to support it. It has its place and also has supported features CSS has only fairly recently begun to see equivalents for (eg: motion paths).
Swapping that out for the equivalent CSS animation and everything was smooth as butter. It's obvious that SMIL performance is not as important to the browser makers than CSS animation performance.