The additional slots added by inventorio(all the off hand ones and the toolbelt slot that comes on the right when you mine something) are flashing and it wont stop. I have this glitch on server and on singleplayer. I made a 1 min video showcasing it but oddly when I started the video, my fps dropped(probably because of my bad computer) and the thing stopped flashing. it also stopped flashing. but the second I stop recording it stops. I think this might have something to do with fps as when I set my max fps to 10 it stoped. and when I set it to 20 it started again. Mabye it's somthing to do about the stability of the fps but I usually play around 30 fps on the low end with some lag spikes so it's odd that it wont run 20 fps at 2 chunks render distance and when I look at f3 it says I'm running at 20fps.
Mabye it's something to do with the other mods I have. Here's a list of them
List of Server side only performance mods:
-C2ME - https://modrinth.com/mod/c2me-fabric
-Debugify - https://modrinth.com/mod/debugify
-FastLoad - https://modrinth.com/mod/fastload
-FerriteCore - https://modrinth.com/mod/ferrite-core
-Krypton - https://modrinth.com/mod/krypton
-LazyDFU - https://modrinth.com/mod/lazydfu
-Lithium - https://modrinth.com/mod/lithium
-MemoryLeakFix - https://modrinth.com/mod/memoryleakfix
-ServerCore - https://modrinth.com/mod/servercore
-Starlight - https://modrinth.com/mod/starlight
-VeryManyPlayers - https://modrinth.com/mod/vmp-fabric
-RecipeCache - https://www.curseforge.com/minecraft/mc-mods/recipe-cache

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Client & Server Side Mods:
-Origins - https://modrinth.com/mod/origins
-Origins: Classes - https://modrinth.com/mod/origins-classes
-Enchancement - https://modrinth.com/mod/enchancement
-Waystones - https://modrinth.com/mod/waystones
-Fabric Tailor - https://modrinth.com/mod/fabrictailor
-You're in Grave Danger - https://modrinth.com/mod/yigd
-More Villagers - https://www.curseforge.com/minecraft/mc-mods/more-villagers-fabric
-inventorio - https://www.curseforge.com/minecraft/mc-mods/inventorio
-Enchantment Descriptions -

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Server Side Mods:
-Forgiving Void - https://modrinth.com/mod/forgiving-void
-YUNG's Better Dungeons - https://modrinth.com/mod/yungs-better-dungeons
-YUNG's Better Mineshafts - https://modrinth.com/mod/yungs-better-mineshafts
-YUNG's Better Ocean Monuments- https://modrinth.com/mod/yungs-better-ocean-monuments
-YUNG's Better Strongholds - https://modrinth.com/mod/yungs-better-strongholds
-YUNG's Better Desert Temples - https://modrinth.com/mod/yungs-better-desert-temples
-YUNG's Better Jungle Temples - https://modrinth.com/mod/yungs-better-jungle-temples

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Client Side Mods:
-Mod Menu - https://modrinth.com/mod/modmenu
-Tool Tip Fix - https://modrinth.com/mod/tooltipfix

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Client & Server Side Libraries:
-Fabric API - https://modrinth.com/mod/fabric-api
-Cloth Config API - https://modrinth.com/mod/cloth-config
-Balm - https://modrinth.com/mod/balm
-Kotlin - https://modrinth.com/mod/fabric-language-kotlin
-Pehkui - https://modrinth.com/mod/pehkui
-Bookshelf -

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Server Side Libraries
-YUNG's API - https://modrinth.com/mod/yungs-api

I suggest removing other mods one-by-one and seeing if that fixes the issue, as neither of the mods listed has previously been confirmed to have such issues.

I'll try it and get back to you. Might take a while though or might just not get done out of pure laziness.

Update: I haven't tested this but I'm pretty sure It's because of exordium which renders the gui at a lower fps than the world or something (that explains why fps was involved in the flashing)