This bug was originally marked as duplicate of bug 171183, 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 Beta (Build 201210011125)
VM: Java HotSpot(TM) 64-Bit Server VM, 23.3-b01, Java(TM) SE Runtime Environment, 1.7.0_07-b10
Maximum slowness yet reported was 3290 ms, average is 3290
Created attachment 126428 [details]
*** Bug 220709 has been marked as a duplicate of this bug. ***
Sounds like issue 171183 hasn't been fixed ...
As can be seen from the attached snapshot, AWT EDT is blocked by an extremely slow class loading. Nothing to fix on the code completion side. Closing as WONTFIX.