Pity that in this rewrite, they were unable to speed up compilation speed. From the latest reports, dotty appears to compile at about the same speed (if not more slowly than) scalac.

Where did you get that from? I’ve seen performance increases of 2-3x mentioned on conference talks: eg here https://d-d.me/talks/scalaworld2015/ slide 50.

This is pure speculation on my part, but my guess would be that better, simpler foundations and a fresh start end up unlocking a lot of optimization opportunities in the long run. But it would seem foolish to invest too much in optimization while things are still in flux.

Scalac has evolved considerably, but it appears to be much harder to evolve. The Scalac team and contributors are backporting some of the Dotty features, but this is far from a trivial effort. Doing the same changes in Dotty is typically much easier in comparison.

This does nothing to change my opinion that total rewrites are almost always a horrible idea.

The Scala compiler has to be cleaned up. Maybe rewritten, maybe not. Dotty started as a fork of Scala, and now Odersky decided/announced that it'll be merged back eventually. (And it should conform to whatever the Scala language is/will be after the SIP processes for 3.0.)

I've never seen a system or project that's had a "big" change that wasn't always a total rewrite.

I guess it's always envisioned differently like "can we tackle this subsystem in isolation".

The old saying: "Its bigger and slower, but at least its more expensive!"

My experience says, a rewrite just has different bugs. Combined with delay and cost, its often not worthwhile.