Keyboard lag on iPhone SE When I use an app called Termius on iOS to SSH into my different machines, I am starting to get serious keyboard lag lately. I’ve already tried resetting the dictionary and I’ve restarted my phone.

Does anyone have an idea what is happening here?

I am running iOS 12.2.
Since you are facing the issue only with a single app, it most likely is a bug with the app, not with the iOS software or the hardware.

The resolution step would be to report the same to the developer of the app. The contract information can be found on the App Store page of the app. You can also try by running the app on other iOS devices if you have access to any.