I can confirm that free space has an impact on performance. I run (or better: ran) Aerofly2 on an iPad Air (1) with 128GB. The game itself ran fine at the beginning (medium graphics settings), still fine with southern California installed, but after buying the airplane package it chrashed almost always (depending on aircraft). Of course I tried resetting, downloading again, and tried all kinds of combinations. Btw: Thanks, Ipacs support. I had about 4GB left on the device.
Then, I cleaned up my iPad so that I had about 11GB free space left. From then on, Aerofly ran almost perfect, also in medium settings.
So, free space seems to have an impact in the end.