Glitching/ being slow/ storage

You may read this Apple Article for troubleshooting your device --> About iOS performance on your iPhone, iPad, or iPod touch ...

You didn't purchase iPhone storage, you purchased iCloud storage. You can't buy more storage on your phone, it is a fixed size.

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