TextMate makes sure that the project you are working on has an up to date file list. One of the ways that it does this is by scanning the directory tree of your project every time TextMate regains focus. This is usually good. However, when working with large projects or a project based on file server via NFS or CIFS, TextMate can take seemingly forever to actually regain focus when switching back to it from another application.
Luckily there is ReMate, which gives the ability to disable the project refresh feature. This option is added under the Window menu and also if you right click on any file inside a project there is a new menu item in there called Refresh All Projects.