After upgrading to OS X 10.7, a number of people who use Apple's iWork productivity suite have found that its Keynote application was having some problems, with one of the big ones being hangs when managing large presentation files. While the program would handle basic presentations just fine, for presentations over a few hundred megabytes in size, the program would slow down and eventually freeze.
Keynote is Apple's alternative to Microsoft's PowerPoint presentation software, and it offers a number of enticing features that a number of Mac users have come to rely on. When this bug happened a number of people were affected (1, 2), and a few were forced to downgrade their operating systems to Snow Leopard so their presentations would run properly.
Recently Apple released version 5.1.1 of the Keynote program, which in addition to other stability improvements is supposed to address the hangs and freezes when managing large presentation files.
For people who have installed Keynote through the Mac App Store, the update should be available in the Updates section of the store, so go there to check for and install it. If you have installed Keynote from the iWork installation DVD or from downloading the trial version of the software and then registering it, then you should be able to get the update through Apple's Software Update utility (in the Apple menu in OS X), or you can download it from the Keynote 5.1.1 update Web page.
The update may vary in size if you download it from Software Update, but the standalone updater is 29.94MB in size. The update requires OS X 10.6.6 or later, so if you have downgraded your system to Snow Leopard for using Keynote, you can install the update now and then consider upgrading to Lion again at a later point.