freeze and latency when battery is <30%

Hello

Since a couple of days, I've been experimenting serious latency and freezes when the battery of my iPhone 6S is getting low (since about 30%) and as it gets lower, it gets worse.

I tried to recalibrate the battey but it didn't help.

If I plug the charger, it immediately becomes fine again (even if the battery is still low).

It is almost unusable, when you type you don't get an immediate feedback as usual, you need to wait some seconds to see what you've typed, for example. Or if you try to launch an app (or navigate Settings) it takes 4-5 seconds to load it, and if you try to take a picture, it will take ages. Plugging the charger immediately cure it.

I'm on iOS 10.3.3.

What could be the reason?