I have used the following tools to to performance/load testing-
Android Debugger tool - https://developer.android.com/studio/command-line/adb.html. You can get various performance metrics by this free tool you can use on Android Apps.
Instruments - If you have an iOS app, I would recommend using Instruments that comes free as part of Xcode. It helps to find different vulnerabilities in your app like Memory Leaks, Page load times etc.
Check out this tutorial - https://developer.apple.com/library/archive/documentation/DeveloperTools/Conceptual/InstrumentsUserGuide/LaunchingInstruments.html#//apple_ref/doc/uid/TP40004652-CH7-SW1
- JMeter is a great tool as well to simulate load on your applications.
These are the tools I would recommend to start with. Then you can explore other tools such as
NeoLoad - https://www.neotys.com/neoload/overview
WebLOAD (Free - 50 virtual users)— http://www.radview.com/webload-download/
LoadComplete - https://smartbear.com/product/loadcomplete/overview/
Apache Jmeter (Free)- http://jmeter.apache.org/
BlazeMeter - https://www.blazemeter.com/
Hope this information helps you out.
-Raj
