What about Minix 3?

Probably the most widely deployed microkernel ever: it's inside every Intel CPU for the last ~20 years.

Already has a mostly-working port of the NetBSD userland.

It was shaping up quite well until Andy Tanenbaum retired. Since then, nothing.

Of course, Intel has never released any of its code, which is terrible -- but legal and license-compliant.

https://www.osnews.com/story/136174/minix-is-dead/

I spent a lot of time with Minix 3 (see https://github.com/rochus-keller/Minix3/tree/Minix3_Book_TCC). The intention was to make the book version compile with TCC. It turned out that this version only covers a fraction of the relevant parts necessary to build a working system. Minix 3 in addition includes hundereds of assembly files. The impression created by the book of a lean C implemented kernel, or the claim "for compiling a small Minix system" are wrong. Not even the kernel is complete; the central IPC is actually implemented in assembler and linked with the kernel via some library not described in the book.

Concerning performance: studies suggest that Minix 3 is significantly slower (10 to > 100 times) than L4 and even Linux (see e.g. https://www.cscjournals.org/download/issuearchive/IJE/Volume... or https://www.minix3.org/theses/priescu_thesis_2012.pdf).

Concerning "inside every Intel CPU": we actually don't know which version that was supposed to be and how much it was modified.

The reason why I omitted Minix is that I don't know about its performance characteristics. Same for TRON / eTRON which is or used to be huge in the Japanese industry.