The values ​​of the Nbt property, which I use to create custom elytras and armor, doesn't work anymore and began to cause a FPS drop.
A possible reason for this could be the asterisk character in the property - in Optifine the asterisk character means any object in the array in json format:

This worked until version 2.2

I use it to identify an item in a particular slot, but the player does not have an "ArmorItems" tag, only "Inventory", and the item can change its location in the array based on the number of items in the inventory.
Mobs has ArmorItems tag and it works fine (because it doesn't use the asterisk)

Removing asterisk may cause detecting these tags in an another object (by another item in any another slot)
10 and 12 - old nbt format, 11 and 13 - 1.20.4 nbt format, just in case.

This also provokes endless spam in the game logs:

• Minecraft Version: Fabric 1.20.4
• EMF version: 2.2

Hmm, I had not considered purposefully wrong nbt in the nbt property rewrite. Such as armor items potentially running on players.

I will look into it, hopefully the log spam is the only source of the lag.

This is from the etf update not the emf one, but keep this issue open anyway.

Unfortunately, but log spam is not a source of the lag. With the updates, no log spam anymore (same message no longer appears), random armor (and elytra) began to be displayed on the player, but game still lags - FPS drops to 0 when the armor is worn on the player.

Example pack: ZeroFpsArmor.zip
Rename Leather Chestplate to Cape