The new borrow checker is not yet all that fast. For instance, it was 5000x slower, according to a recent report.

https://users.rust-lang.org/t/polonius-is-more-ergonomic-tha...

>I recommend watching the video @nerditation linked. I believe Amanda mentioned somewhere that Polonius is 5000x slower than the existing borrow-checker; IIRC the plan isn't to use Polonius instead of NLL, but rather use NLL and kick off Polonius for certain failure cases.