I cannot enter a password in my iPhone and it's really slow

There may be several reasons due to which an iPhone may behave as described by you. Some of them could be...

1. Storage could be an issue --> How to check the storage on your iPhone, iPad, and iPod touch
2. Battery health could also reduce performance --> iPhone Battery and Performance - Apple Support
3. Incorrect handling of the iPhone --> Important handling information for iPhone - Apple Support
4. Dust ingress into the iPhone - Cleaning your iPhone - Apple Support
5. Water / Liquid entry into the iPhone---> Important safety information for iPhone - Apple Support
6. Overheating due to exposure to the temperature beyond -20º and 45º C (-4º to 113º F). especially in the car because temperatures in parked cars can exceed this range -- Keeping iPhone, iPad, and iPod touch within acceptable ...
7. Service of an iPhone by an un-authorised service personnel / agency / store
8. Modification / Alteration of an iPhone by a third party vendor
9. iPhone is Jail Broken

asiskande ~ Does your iPhone XR still have its original battery?

And, if the screen allows, what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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