One new feature in this release is that PCL can now use nanoflann, which is a faster alternative to FLANN for neighborhood searching. To find out how you can speed up your ICP, normal estimation, etc, please see here.
Of course, this release also contains a variety of speed improvements, fixes for the newest compiler versions, and more. For an exhaustive list of newly added features, deprecations and other changes in PCL 1.15.1, please see CHANGES.md.