To improve the performance of WordPress, I simply removed all of the unactivated plugin files files from my plugins folder. It’s hard to tell exactly how much this improveds things, but page load times are definitely a little quicker.
I had several WordPress plugins that weren’t activated but were in the plugins folder. Here’s the list: wp-db-backup.php, backuprestore.php, pagenavi.php, wpPaginate-v2.php, suggest.php, subscribe2.php, search-meter.php, popularity-contest.php, af-extended-live-archive.php, words_in_post.php, hello.php, wysiwordpress.php, textile1.php, markdown.php, enhanced-post-list.php, did_you_pass_math_functions.php, did_you_pass_math.php, challenge.php
Phew, I play a lot with WordPress plugins :-)