I realise that your budget or professional needs may not support it, however I've always found Keynote to perform much better than PowerPoint.
Download and install the latest Office update, released today, it might make a difference if your version of Office is old:
Office 2004 11.2.3 Update
Also make sure you've kept on-top of all the OS X updates, which often include QuickTime and graphics performance improvements (Apple Menu > Software Update).
Matt