Mandoc is a tool specifically for formatting man pages. Groff is a general typesetting tool. That probably accounts for most if not all of the difference in speed.
Probably a good choice. For a workstation, you might encounter the need for a typesetting system. But for your router, you probably don't need it. So I think OpenBSD made the right choice about what to ship in core.
troff is technically a general-purpose typesetting system, but how many people are still using it for anything other than formatting man pages? (La)TeX and other formats have pretty much taken over.
Indeed. I hope Linux distributions will follow with replacing Groff. It's a huge, slow, ancient system and for the purpose of formatting manpages it can be replaced by a 15-line Python script.
(this has nothing to do with C++ versus C, simply with removing some bloated UNIX legacy code)
