I've been using Lussumo Framework quite frequently (as some of you know!) and I've got a few optimizations that might be worth examining. This is a list I've been compiling so that I don't start a bunch of unnecessary discussions about related subjects.

1. Use $_REQUEST in ForceIncoming* functions
   This one seemed pretty obvious to me at first but then some hurdles came to mind: People can customize the variables_order. It just so happens this setting cannot be changed by Vanilla without the help of .htaccess like php_flag variables_order EGPCS. However, I still think the benefits outweigh the sacrifice. This will create a smaller Framework Functions file and be faster as it is using a variable that PHP already populates on every request.
2. Remove '@' error suppression from ForceIncoming* Functions
   I've read in various places that '@' suppression is slow. I have no thoughts or tests to back this one up, anyone have any thoughts on this? In combination with #1 above it would be fairly simple to remove '@' suppression and hopefully speed this up a little more: $iReturn = isset($_REQUEST[$VariableName]) ? $_REQUEST[$VariableName] : $DefaultValue;
3. Some functions still use global $Configuration;
   These functions should receive the Configuration as a parameter: ConcatenatePath(), FormatStringForDisplay(), GetRequestUri().
4. FormatHyperLink() contains extra strpos() checks
   This might've been intentional but I don't think it is necessary to check strpos() for each condition: if (strpos($InString, 'http://') === 0) {

I might have some more as I continue working but this should be enough to get the conversation started :)