Nether portals break in a ripple pattern, causing performance issues

using a /setblock portal before it dissappears will also sometimes freeze the game, crash it or stop rendering blocks on the other side.

edit: wasn't able to replicate it, but you get similar effects when going into a nether portal tile from the end to the overworld.

the problem appears to occur due to portal blocks now using pendingTicks rather than instant updates

This breaks the gold / xp farm mechanics too, refer to following link:

.  I think it relates to MCPE-98111 Sugarcane, Cactus, and Bamboo use Instant Updates. Still broken in Windows 10 PC v1.16.100.56 Beta.

The change to portal tile behavior is quite inexplicable from the player community's point of of view. The stated reason for the change in the 1.16.100.55 changelog is that filling a large area with portal tiles at once with a /fill command could cause a crash.

Fixed crash when using /fill command with Portal to fill huge amount of blocks

This sure sounds like just a special case of MCPE-94942, which is reproducible with a number of blocks. There is no apparent reason why portal tiles were singled out for a special fix attempt. Moreover, the description of MCPE-94942 had already pointed out that

This cannot simply be fixed by making everything use pendingTicks, since that is simply not possible for blocks like doors, beds and portal tiles without adding some dupe glitches or wierd behaviour.

(emphasis added)