I have 2 big problems with the CMS i'm doing (we developed our own CMS)

1- We need to optimize it a lot because 1 of our client said that after about 30 min on Internet Explorer 8 the CMS is lagging and she must close the browser and reopen it.

2- The same client send us a very strange erreur I never saw:

Method not implemented
get to /index.html not supported
Additionaly, a 404 not found error was encountered while trying to use an ErrorDocument to handle the request.

Our CMS use PHP, ajax, jquery, prototype.
Can you please tell any optimization methode (ajax, jquery, prototype, method to use ans not to use, ) and any theories/solutions for both problem

Thank you