The best way to know this is to run a benchmark yourself using a workload that is relevant to your problem.
One important thing to remember is that Dart VM uses JIT so when benchmarking you need to give it a chance to warm-up to get to its peek performance.
From my personal experience: Dart VM is fast. Don't know if it's the fastest but it's fast enough for many scenarios.
Plus it gives you async IO which makes writing IO heavy apps a lot easier and execution a lot more efficient.
Plus it allows you to utilize multiple CPU cores using Isolates. Some built-in classes like WebServer even provide
a convenient way to leverage those.
This makes your setup a lot simpler comparing to PHP where you'd need an OP cache extension, NGINX as a web-server, PHP-FPM, etc.
With Dart you just need the VM.
In any case, if you end up creating a benchmark it would be great to see your results!