+=item PERL_INLINE_DIRECTORY
+
+Pointing this to point to a writable directory enables the use
+of L<Inline> and L<Inline::C> extensions which may provide
+platform-specific performance improvements. Currently, this
+enables the use of L<vfork(2)> which speeds up subprocess
+spawning with the Linux kernel.
+
+public-inbox will never enable L<Inline::C> automatically without
+this environment variable set or C<~/.cache/public-inbox/inline-c>
+created by a user. See L<Inline> and L<Inline::C> for more details.
+