I'm going to come at this from a slightly different angle
I mean this by not ever since they stopped using hard drives in devices, so not mobile phones then. Hard drives would slow down the read/write speed as you filled them up only due to you filling up the platter and the head having to travel further to read/write the data. Hence defragmentation back then and the tedious bordem it entailed.
That is not the case in SSD as far as I am aware.
However if you were to fill your phone to the absolute brim it might slow down ever so slightly as the OS may use some of the SSD space as a memory swap partition to keep things going when RAM is being pushed.