I was speaking past tense; that company was sold. But in general big compiling tasks can be run in parallel, so more computers can speed it up. If you want to compile for PPC but not on PPC, cross-compiling should also work. The latest 500-core, AI infected whatever-majig could probably do it faster than a stack of Minis. So I guess my point is, you could get more old Macs to speed up compiling or else cross-compile with something faster.
yewtu.be/watch?v=eO33U-qqfME