Hey marton210,
Since restarting can resolve issues such as this, restart by following the steps below:
1. Press and hold either volume button and the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
The steps are also available in this article: Restart your iPhone
If you do not notice any improvement, the information here may help: About iOS or iPadOS performance on your iPhone, iPad, or iPod touch. The article provides great tips regarding the issue you're experiencing.
Take care.