How old is this iPad? iPads that can no longer update past iOS 12 are considered vintage or even obsolete by Apple themselves. See this page for an easy overview!
We cannot possibly support this old hardware without limiting ourselves, and support for iOS/iPadOS 12 will be dropped in the not-too-distant future. The list of features included in our app will only grow, and with it comes the need for more device resources.
For the Android tablets, the issue could be just about anything. That’s the issue when the market consists of tens of thousands of different models 
Things like specifications (of course), hardware acceleration, overheating, Android versions, installed apps, bloatware etc. all vary greatly, which makes it impossible to make something that works well for everyone.
A good example of this is that one of our team members uses a Lenovo M10 (as mentioned earlier in this topic), and he does not have any issues with it at all: Charts V8 Performance - #9 by NAVData.
It’s impossible for us to know what the issue could be since we can’t even reproduce it ourselves with the same hardware. At that point, it becomes a question of bloat and malware, which has been more common on Android historically.
While we are doing our best to keep the app as performant as possible for everyone, the thing mentioned above also goes here: we can’t let these factors limit us from developing new features. We would much rather drop support for really old hardware, and based on the statistics that we get from the Google Play store - the majority of our users have no issues with the apps.
As of the current moment, there is no general perception of the Charts application being “extremely slow” on all Android devices. We can’t reproduce anything like it on our test devices, and it is not feasible for us to provide one-on-one support for everyone that has a slow android tablet.
Kind Regards,
Malte