If you remember some time ago I did some tests on the IsolatedStorage and the speed was not very impressive (it was 127 slower than the FileStream). In the new beta version there is a TREMENDOUS improvement. I rerun the old test in the new emulator and these are the results:
Dividing the two values 0.0632/1.1866=0.0532 which is almost the same with FileStream. At this point it does make sense implementing/porting a database engine on IsolatedStorage. Well done WP 7 Team on this aspect.