When "Enable Optifine emissive entity textures" is "On" it reduces FPS depending on the number of resource packs enabled.
No resource packs: ~200FPS
5 resource packs: ~140FPS
25 resource packs: ~40FPS
Resource packs could be even for GUI only.
Long loadings are due to 1.19 bug. Also forgot to mute the mic, sorry.
https://www.youtube.com/watch?v=H5_l3cEk3Ns

are there any errors in the log? i have an idea of how i might be able to alleviate this

latest.log
Removed unnecessary mods. 25 recource packs log.

entity_texture_features_fabric-4.0.2.beta.1.zip
give this a go tell me if it helps

Sadly no. FPS a little bit better. 65 instead of 40, but without "emissive entity" I still have 240
UPD: FPS is better because I updated my GPU drivers. RX570 4GB if it helps.
latest.log
.

i'm not getting the same kind of drop perhaps its the specific resourcepacks?
can you install the spark profiler mod https://spark.lucko.me/ and run "/sparkc profiler" during the lag and after a few minutes run "/sparkc profiler --stop" it will give you a web url you can paste here and i can see exactly what code is causing the lag

also try this build
entity_texture_features_fabric-4.0.2-GITHUBFIX.zip

No changes with new build. https://spark.lucko.me/4MwWHAHRyp
Maybe you can reproduce it using the same map? https://www.planetminecraft.com/project/restworld-a-complete-resourcepack-testing-world-for-1-15-2/ Go to the shield/pattern room.

i will give it a go tomorrow, is it specifically the shield pattern room only where this happens?

does the issue go away if you disable shield emissive textures under special case emissive settings? perhaps it is a shield specific issue.

Yes, it helps. I disabled shield emissive textures and everything works fine.
UPD: I should tell you about map earlier, sorry.

all good mate these things happen, i will investigate this shield issue :) explains why I couldn't reproduce

this should be greatly diminished in release 4.0.2 can you confirm?

Everything works fine. Thank you.