do you have this issue resolved? If so I am curious on your resolution of this.
You can check the OnDemandScanLog.txt to see if there is a preconfigured task that is running outside the one that you created to run at 10. Also it will tell you if the scan is hanging on any large files, if you discover any you can set the timeout to a lower number.
Do you have the high-default-low risk policies set?
If you do then stick the liberty 4 process into the low risk bucket and see if it improves performance.
also make sure you follow all the recommended exceptions for MS servers and IIS etc... that you might be using. This normally helps performance as well.