Jumbo is a unity build system implementation for Chromium aimed at dramatically reducing the build times for the project. (See https://chromium.googlesource.com/chromium/src/+/master/docs/jumbo.md )

One set of source comes from the proto_library template. Most often it's very small targets so jumbo can't do much but it's a game of "one CPU minute at a time".

Locally, it seems that making proto_library use jumbo reduces the CPU usage by about 2 CPU minutes (0.3% of the total on master), resulting in about half a minute faster builds. YMMV