I hope that this is temporary to make working on the project easier, Go can get pretty close to C in terms of speed and it's usually easier to jump into a Go project. But 10 times slower seems way too much.
Indeed -- that was before final optimization work on https://github.com/johnkerl/miller/pull/786 et al., thanks to which Miller 6 performance is now on par with Miller 5 for simple processing, and far better than Miller 5 for complex processing chains. See that PR for tabulated processing-time results.