I have been using this plugin for some time now. Just now I stumbled upon something that it seems I have overlooked in the past.
The file "/includes/plugins_usercountd.php" gets parsed every time postbit_display_complete is called. Meaning that if you look at a thread with 50 posts per page that file will be parsed 50 times. Luckily the script includes no database queries

This has probably little effect if you use a php opcode cache like xcache, apc etc. On systems that don't use an opcode cache for php this plugin
will generate plenty of unneeded overhead.
I'm seriously thinking of removing this plugins from my forums because of this even if it didn't visibly affect me so far (probably because I have always used xcache). My board recently hit the 1 million post milestone so every bit of gained performance is valuable from now on.