One efficiency problem in dokuwiki, especially on systems with shared hosting (so doku's php is less likely to be cached at any given moment), is the time it takes to read all the php files. This is worse when there are a lot of plugins. The current coding standards actually make it worse, but requiring four spaces instead of a tab. Reading all those spaces takes time in an interpreted language!
Compress all php scripts, including plugins. This involves removing comments and spaces. Additional optimizations would be possible, but difficult. Maybe there's a free third-party library that could be installed.
I don't know how much time this would save, but it would probably split the size of the code in half, which certainly can't hurt.