When this scratch file buffering is turned on with Mac OS X v.10.3 or 10.4, the operating system periodically pauses execution of Photoshop for up to several seconds at a time. This can cause problems when painting. Installing the plug-in causes Photoshop to disable the buffering so the pauses do not occur. It also means that Photoshop only takes advantage of the 3.5GB of RAM it can directly access, so performance with very large documents will decrease.
For this reason, we recommend that if you use very large documents and have no problems with pauses during painting, you should not install this plug-in.
http://www.adobe.com/support/downloads/detail.jsp?ftpID=3337
にある文書「Disable VM Buffering plug-in」より引用しましたが、ここを読む限り、プラグインをインストールすれば全てが解決という訳には行かないと思います。インストールしない方が良い場合もある様です。