You didn't say what format you need the window burn in.
But, you could export the timeline to a QT file, drop that into Compressor and use the TC generator in Compressor. And then if you have the Quick Clusters enabled so you are using all your cores, then it should be faster.
Scott Sheriff
Director
http://www.sstdigitalmedia.com
"If you think it's expensive to hire a professional to do the job, wait until you hire an amateur." ---Red Adair