I hear that your iPhone X is running slowly when using the app switcher. I'd be happy to help out.
In this situation, I have a few articles and steps that will isolate and possibly resolve the issue:
Restart your iPhone, iPad, or iPod touch - Apple Support
If the restart doesn't resolve the issue, continue with restoring as new after backing up:
How to back up your iPhone, iPad, and iPod touch - Apple Support
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
Cheers!