iPhone lag

This surely is no due to iOS 14.4.

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

1. Battery health could also reduce performance --> iPhone Battery and Performance - Apple Support
2. Incorrect handling of the iPhone --> Important handling information for iPhone - Apple Support
3. Dust ingress into the iPhone - Cleaning your iPhone - Apple Support
4. Water / Liquid entry into the iPhone---> Important safety information for iPhone - Apple Support
5. 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 ...
6. Service of an iPhone by an un-authorised service personnel / agency / store
7. Modification / Alteration of an iPhone by a third party vendor
8. iPhone is Jail Broken

My phone is less than a year old, and I'm having the same issues. Didn't think about recent update maybe causing the issues. I'm looking for a fix as well. Thanks