There are many reasons as to why an app on a phone might not be running as smoothly as it should. It could be that the phone’s hardware is old and there might not be enough RAM to handle the app properly. Or maybe it could be that the app was not designed properly and not optimized, hence potential lag.
WALT is actually a tool that is being used by Google themselves when it comes to developing their own apps, as well as for other products like Chromebooks. However Google has decided to make it open source so that other developers will be able to take advantage of it. Unfortunately the only downside is that you will need to build separate hardware in order to make full use of it.
The upside is that according to Google, purchasing the parts necessary to build your latency-testing machine will cost less than $50, so if you’re motivated enough to try and optimize your app as much as possible, then you will be able to find the code and the instructions on GitHub.