Found a changelog:
Performance and quality of service improvements
We’re always working to improve the performance and quality of our services and we’ve made significant progress in this area in today’s beta update. This includes things like decreasing the time it takes to start each program or render each webpage, and improving the quality of your experiences. It often takes hundreds of small improvements and optimizations to better the final experience. Here are some of the many improvements you’ll see in this area:
A quicker Messenger – The time it takes to sign in, and to refresh contacts and social feeds, as well as animation speed, are all faster than in previous versions of Messenger.
More efficient video chat - Messenger video chat uses 30% fewer CPU resources by offloading work to the GPU.
Better facial recognition - Photo Gallery facial recognition is improved significantly and works more quickly.
Larger movie uploads - Movie Maker will upload higher resolution movies to SkyDrive (480x640 vs. the previous 320x480).
Higher bit-rate movies - Movie Maker now supports higher quality (bit-rate) content.
Better spell-checking - Writer has significantly improved the quality of its spell-checking.
Better integration with Office - Writer is much better at retaining all formatting when you copy and paste from Word and other Microsoft Office programs.
Better handling of Gmail – Mail now automatically handles Gmail’s spam and trash folders properly.
Faster web filtering - Family Safety web filtering is 35% faster than in the previous version.
In addition, we’ve been able to improve the quality of the software and have fixed over 75% of reported crashes. We’re not always able to reproduce all of the reported crashes internally, but we do look at this on an ongoing basis.