I am unsure whether or not this issue was already pointed but regardless I'm still going to give my report. like the summary says, the performance on my device has dropped by a lot when updating to 1.19.

Specifics:

-Chunks load a lot slower. After testing on my iPad Air 3, chunks on average take around 2-3x longer to load on 1.19 while on the previous version 1.18.31, loaded the chunks almost instantly. Likewise, this has also caused menu's to also load slowly as they won't open until the chunks load.

-Entities cause a lot of lag. The game starts to slow down when there's more than 10 players on screen. This is an issue especially on PvP based gamemodes on servers such as Cubecraft and Hive.

-Particles also cause noticeable lag. Although this one is pretty minor, if you take the other reasons into consideration, adding more lag on top makes the game feel worse to play.

In my opinion, the edits that Mojang has made to the Render Dragon engine going from 1.18.31 to 1.19 was unnecessary. I do have some suggestions that I would like to share if it's not already being worked on. 

Basically, all platforms should have slightly different coding structures to Render Dragon that is best optimized for their device. E.x, Render Dragon on iOS should be using the same code from 1.18.31 as I think it had the best overall performance for every device on that platform. 

I understand if my suggestion won't make the cut due to it probably needing a bigger workload, however, I would still like the performance issues that come with 1.19 to be fixed or at least improved upon. Please don't overwork yourselves trying to fix this, take as much time as you need.