It happened once or twice that when I was opening many files and projects simultaneously the Netbeans IDE started to become irresponsive.
sudo gedit /usr/local/netbeans-6.8/etc/netbeans.conf
Netbeans hang on open more files, trying to fix that…
netbeans_default_options="-J-client -J-Xss2m -J-Xms96m -J-XX:PermSize=96m -J-XX:MaxPermSize=384m -J-Dnetbeans.logger.console=true -J-ea -J-Dapple.laf.useScreenMenuBar=true -J-Dsun.java2d.noddraw=true -J-Xverify:none"
Save and restart Netbeans.