How much can I fill up my iphone 5 32gb before it slows down?

How much can I fill up my iphone 5 32gb before it slows down?

You can fill it all you want and it won't "slow down" the speed of a device is influenced mainly by how many background processes are running.

Filling up storage of any kind will on fact slow down the device. The operating system runs on the disk and the more that is stored on the device, the more information the device needs to filter through. With a 32 GB iPhone (same as I have) about 25GB or so is probably as full as you would want to go. That is of course an estimate but I have filled my device before and around 25GB I started to get smoother performance.