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.