The solution: change the Java version used and increase the allocated memory.
Edit eclipse.ini – this file is located Eclipse.app/Contents/MacOS/eclipse.ini.
Change the following vars to use Java 1.6 and allocate more memory:
-Dosgi.requiredJavaVersion = 1.6
-Xms128m
-Xmx1024m
(Found originally here: http://www.codeweblog.com/eclipse-adt-mac-version-tab-to-switch-slow-solution/)