The 20 minute talk is posted here. I spoke till the speaker timer started to flash at me (giant flashing clock), which resulted in having no time for a forum Q&A, but feel free to post your questions/comments down below. I'm always curious about what perf issues people are hitting.
Understanding perf issues gives me an idea of how people are stretching the platform beyond our key scenarios, and which APIs/codepaths we should improve in the next version.
Tim Sneath wrote an excellent writeup of my 20min mix session on writing an optimized SL app. I almost wish that I had his post to read off as a script!
Talk Video & PPT: http://videos.visitmix.com/MIX09/T67M
Tim's great summary: http://blogs.msdn.com/tims/archive/2009/03/24/building-an-optimized-graphics-intensive-silverlight-application.aspx