Hello,

I think you can also ask this in joomla forum about issues with php5. I think it is designed for php4 at the moment. Why would you need php5? I would enable it only if you use a script that requires it, otherwise just keep php4 this is the best for joomla.
For the speed of your joomla it depends on a lot of things. Do you have an SEF installed? (or the one that comes with joomla?). If yes, this slows your site and you could think about using SEFadvance that does not slow your site. But you have to pay for it.
There is also in the extensions part of joomla website an extension or plugin/mambot that improves joomla cache to make your site more rapid.
Also try to stop any kind of statistics or other things that use sql queries, this slows your site...

oh btw, when did your site begin to be slow? are you sure this is not a problem with your server? What did you install or do before it becomes slow?

good luck and tell us if you find what works best for you in the end ok?
cheers

Sorry I do not experience yet in configuring the cache but I will look into it soon as I also want to imrpove the rapidity of my website

good luck, hope others can help as well