Ikiwiki has already been optimised a lot, however..

* Don't render blog archive pages unless a page is added/removed. Just
  changing a page doesn't affect the archives as they show only the title.

* Look at splitting up CGI.pm. But note that too much splitting can slow
  perl down.

* The backlinks calculation code is still O(N^2) on the number of pages.
  If backlinks info were stored in the index file, it would go down to
  constant time for iterative builds, though still N^2 for rebuilds.