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.
With that said, I recently saw some mention on Zulip about some substantial performance gains, and I'm pretty sure it was relating to Polonius. So maybe that is no longer true?
Anywho .. the short version is that Polonius isn't finished yet. Going by the Polonius stream on rust-lang Zulip, it has periods of high activity, and periods of very low activity. It's definitely not dead, but I think it's in a "we definitely want this, but clearly people are being highly productive with Rust without it, so we don't need it tomorrow" place.