+The most time consuming task for building the documentation is running
+LilyPond to build images of music, and there cannot be several
+simultaneously running `lilypond-book' instances, so `-j' `make' option
+does not significantly speed up the build process. To help speed it
+up, the makefile variable CPU_COUNT may be set in `local.make' or on
+the command line to the number of `.ly' files that LilyPond should
+process simultaneously, e.g. on a bi-processor or dual core machine