Took a while to find on their website but here’s a benchmark vs AWS Aurora:

https://planetscale.com/benchmarks/aurora

Seems a bit better, but they benchmarked on a kind of small db (500gb db / db.r8g.xlarge)

Fair to say 500GB is small, especially compared to some of the folks who've already migrated, but do note that it's 15x RAM on the benchmark machines, so we really were testing the whole database and not just the memory bandwidth of the CPUs.