My iPhone takes time to process photos after taking them

Kajisaw ~ With your iPhone XR possibly being nearly six years old, it's worth checking for a weak, aging battery which can cause various problems. So...

Does your iPhone XR still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health & Charging

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Let's gather some information that may help diagnose your issue.

Did this suddenly start happening, or has it been getting worse over time?

If it happened suddenly, was there some triggering event, like a system update?

How full is the storage on your device?

Do you have iCloud Photos active on your iPhone XR?

What version of iOS are you running now, and what version were you running just before the problem started?

I'm not saying that there is any correlation, but it is plausible that some update could have introduced a processing step that takes too much time on the older processor on your XR.