This bug was originally marked as duplicate of bug 219038, that is already resolved. This bug is still valid, so this seems to be another bug, but it might be related.
Build: NetBeans IDE 7.3 (Build 201302132200)
VM: Java HotSpot(TM) Client VM, 23.7-b01, Java(TM) SE Runtime Environment, 1.7.0_15-b03
GUEST: editing gui
Maximum slowness yet reported was 24099 ms, average is 24099
Created attachment 132794 [details]
Method o.n.m.form.JavaCodeGenerator$FormListener.formChanged() performs some operation (in EDT) that requires class loading.
This probably cannot be fixed. It seems that the system was under heavy load.