Ikiwiki has already been optimised a lot, however..
Look at splitting up CGI.pm. But note that too much splitting can slow perl down.
It's split enough, or possibly more than enough, now.
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.
Seems to be O(Num Pages * Num Links in Page), or effectively O(N) pages for most wikis.