Using secure mode (
https://) to serve up pages in "http://" will cause a noticeable reduction in performance. It's bad practice.
Here's a trick I use to avoid the "mixed content" message.
When linking to images or files that are used in both the secure and non-secured pages, try using relative URLs.
Here's a page outlining some additional issues to look out for.
IE Mixed Content - Page Contains Both Secure and Nonsecure Items | Zorked!
I'm pretty sure you're using include files, so if you want to specify an absolute path, you might want to use a script to check for a secure or non-secured session. If it's secured, just append an "s" to the "http".
.02