Query Performance Improvements
1.0.9 contains query performance improvements that should lead to slightly better database performance for Joomla! sites.
There is roughly a 20% reduction in number of queries called in 1.0.9 compared to 1.0.8
In all earlier versions of 1.0.x the core caching functionality is basically does not work, this has now been corrected in 1.0.9
This means that activating Joomla! caching should now lead to actual performance improvements.
MOSImage Performance Issue
There is a performance issue in regards to the MOSImage Manager that can slow the loading of the Content Item create/edit page when you have a large number of images in the /images/stories/ directory.
To solve this, new functionality has been added to 1.0.9
Losing data by getting Logged out
One common problem in the 1.0.x series is getting logged out (because your session has expired) and losing the data you had been working on. This has been addressed in 1.0.9 by a slight change in the logout system. This should mean that the circumstance of losing data due to being logged out will be a thing of the past.
Language File changes
In 1.0.9 modifications were made regards the frontend static language file. For non-english Joomla! users (users who use a different language file for their site instead of the default english.php) this may cause some minor issues.
Until the 3rd Party Development team in charge of creating your specific language file has updated their language versions, then you need to make the changes in your language file yourself.
Other fixes & improvements
Any other new improvements in 1.0.9 will be blogged about in the Official Joomla! Developers Blog.