Sierra

In my case, the command block is in Needs Redstone, so don't expect Always On setting to be turned on

ive tested both setups listed here and i have found no noticeable TPS drops, measured both with server software and with snowballs. The problem probably lies with how powerful your device is, while lower end devices might struggle with this, other machines probably will not have a problem. Therefore, i truly wonder if this a bug or just poor device performance.

ah yes a very informative report

In the changelog MCPE-65762 was mentioned as the "inspiration" for this change, and I think that bug report is more of a personal issue. So if that bug report is what you want to address, maybe have each difficulty spawn different amounts of mobs so players with the want to play with less mobs can do so without effecting the entire community. With this implementation, people who feel that mobs spawn to frequently can play on easier versions with less mobs, while other players can play with a larger mobcap. This is also would be a small parity change as the rate of which mobs spawn on Java Edition changes per difficulty level. MCPE-65762 also seems to heavily lean on bugs such as MCPE-46540 where mobs spawn in smaller spaces than intended, so people would also feel less crowded with that bug fixed.

i have the same issue, i can give my resource packs if needed but demonstrated by these videos the game actually creates a ghost stone block that impairs vision 

https://streamable.com/o45kcf

https://streamable.com/iiv6y7

Oh oops I didn't know that, I honestly thought I was still in survival mode but I do remember turning creative mode on to grab night vision potion to help me see on the way down, but forgot to switch it back. I had a DUH moment lol it happens from time to time. 😅