Render distance only 6 on iPhone X
I think there are one of three problems going on here(or even a combination of two or three):

1. Minecraft is not able to fully utilize the 6-8 cores of a modern mobile cpu and instead can only use 1-3 cores.
2. Minecraft’s ram management is broken and/or needs major overhauls.
3. (Sadly the most likely) Microsoft is intentionally limiting the mobile performance of the game so that phones cannot match or beat the Xbox ones 8-14 render distance. This would make sense as mobile cpus like the A11 and SP835 are multiple times faster than the 2012 midrange desktop cpu in the Xbox one. This to keep console sales up Microsoft limits mobile devices to 6-8 chunks.