>But is the Xcode.version/CLT.version stuff going to slow things down if it's in the install method?

No, because when it's in the install method it is only evaluated when the method is called. When it's in the class body, it's evaluated when the file is loaded, so any otherwise "fast" commands will be slower.

Determining the CLT version can hit the filesystem pretty hard, so on older systems (without SSDs) it can easily add 1-2 seconds of overhead. The same goes for `X11.installed?` and other things that use pkgutil or mdfind.