No computing devices can work properly if they are close to being filled with data!
iOS needs an absolute minimum of 2-3 GBs of free data storage space to function nominally or with near average performance.
If an iDevice can maintain closer 4 GBs, or so, iOS performance should return to more normal, expected levels.
Many users run their computers and mobile devices down to the point where the operating systems have NO amount of data space to function properly!
Also, mobile devices have limited RAM.
Users who leave scads and scads of apps in the Background app switcher that are still running active in the background and constantly grabbing up to date data from the Internet, eats of RAM, CPU cycles and other resources, like your battery.
AND to add insult to injury if you leave a web browser open with scads and scads of active website tabs, ths eats into RAM and CPU cycles and another hit on battery life!
So, it’s important to not max out available hardware, software and data storage resources of any computing device!