Here are the improvements.
One thing to notice is that your Ext3 filesystem might get slower after upgrading. This is because the Kernel now has implemented the Red hat policy of turning on filesystem barriers by default in Ext3:
Hard disks have a memory buffer were they temporally store the instructions and data issued from the OS while the disk processes it. The internal software of modern disks changes the order of the instructions to improve performance, which means that instructions may or may not be committed to the disk in the same order the OS issued them. This breaks many of the assumptions that filesystems need to reliably implement things like journaling or COW, so disks provide a "cache flush" instruction that the OS uses when it needs it. In the Linux world, when a filesystem issues that instruction, it is called a "barrier". Filesystems such as XFS, Btrfs and Ext4 already use and enable barriers by default; Ext3 supports them but until this release it did not enable them by default: while the data safety guarantees are higher, their performance impact in Ext3 is noticeable in many common workloads, and it considered that it was an unnaceptable performance regression to enable them by default. However, Linux distros like Red Hat have enabled barriers by default in Ext3 for a long time, and now the default for mainline has been changed aswell.
In other words: if you use Ext3 and you note performance regressions with this release, try disabling barriers ("barriers=0" mount option).
As you can see it will not affect XFS, Btrfs and Ext4.