Specifically, the developers tell us that the app has been optimized specifically for all those devices with Android version between 4.0.3 Ice Cream Sandwich and 5.1. Given that these versions are dated, it is reasonable to expect that the performance of the devices involved is also more limited than today's standards.
In fact, Microsoft has reduced the weight of the application and the consumption of memory, allowing to increase the speed and quality of audio and video for more "antiquated" smartphones. A news that, although it concerns versions of Android "old", still affects a large share of users (according to the Android distribution in February the percentage of smartphones involved is 42%).

The update will be released in the coming days, but it will be a gradual release, so do not expect to receive the update in a very short time (Microsoft speaks of "weeks"). In case you do not want to wait and be among the first to receive the update, you can request to enter the Insider program to receive the non-definitive versions of the app.