The chart showing jxl-rs taking 1209ms to decode a 5456x3632 image in a single thread is a little damning, but is there an equivalent benchmark showing multithreaded decode performance? Clients tend to have an abundance of threads these days.
The JPEG XL report [1] measured between 240-270 Megapixels/s on 6 cores using the C++ implementation (disclosure: I was responsible for its SIMD/threading), about twice as fast as the then-current libaom.
Measuring on a single core is deeply misleading because our code was designed to scale well. I believe AVIF requires tiling in order to parallelize, which causes artifacts at tile boundaries.
CPU power is proportional to frequency^2. Running on 4-6 little/efficiency cores (which are widespread on mobile) is likely faster than one big core, and uses less energy.