I've noticed slowdown on my worlds as well (PC version with i7-7700, GTX 1070, 32GB DDR4 memory)
I did see reports some time ago that suggested this slowdown was being caused by mobs spawning in caves then not being despawend as they picked up an item.
I can confirm the achievement does not unlock for me either. Tried multiple goats in different oceans and rivers to no avail.
Basically it's server tick lag, and due to bedrock having an almost always online nature, you constantly have a server running (you have a similar server running when playing single player). i have noticed the game feels way too slow compared to Java, even if my FPS is higher. Whatever the reason is, it's probably tied to the same thing that stops us from being able to pause a world.
Can confirm. Crashed on both IOS and Windows 10. Crash occurs from main menu as well as the pause menu, and occurs regardless if a pack is active or not.