Good news: the reference software (http://gitlab.com/wg1/jpeg-xl) can decode about 50 Megapixels/s on a single Skylake core, with 3.1-3.7x speedup on 4 cores.
Encoding throughput is configurable from ~1 MP/s to about 50 per core. That's somewhat slower than JPEG, but can be faster with multiple cores.