Bad performance for difference function

[Google Code Exporter]

Currently the overlay-difference's performance is very bad, I have to delete 
unnecessary features manually. Comparing to this, Arcgis provide a function of 
overlay->erase function, which is much faster.


What steps will reproduce the problem?
1. use the function difference to delete large set of scatters.

What is the expected output? What do you see instead?
normal output, just low performance

What version of the product are you using? On what operating system?
Qgis 1.9 master-exported on kubuntu 12.04 64bit

Please provide any additional information below.

Original issue reported on code.google.com by [e-mail] on 18 Jul 2013 at 2:09