Severe Persistent FPS Drop in Fabric Modpack After Repeated Use — No Recovery After Relaunch

[Kennxn]

I’m experiencing a major and persistent FPS degradation when using Fabric modpacks. The issue is not related to a single gameplay session or gradual in-session performance loss. Instead, after using the modpack a few times (or sometimes even just once), the performance never returns to initial levels—even after completely closing and restarting the Minecraft instance.

This is not a case of in-session memory leaks. It behaves as if some cache or retained state within the Fabric environment or related mods permanently degrades performance across launches. Cache has been deleted, to no avail.

SYSTEM INFO:
CPU: AMD Ryzen 5 7600X3D
GPU: AMD RX 7600
RAM: 32GB DDR5
OS: Windows 11 (fully updated)
Display: 165Hz, 1080p

Fabric Environment:
Minecraft Version: 1.21.5
Fabric Loader: Latest stable version as of June 7th, 2025
Fabric API: Latest version
Modpack: Simply Optimized (Modrinth) — slightly modified with Continuity and Essential Mod.
JVM: Default Prism Launcher

This issue has happened multiple times across many different instances, across different launchers(Prism, Modrinth, MultiMC)

Issue Details:
First launch after installing: FPS is excellent, consistant 1700-2000 fps.
After a few play sessions FPS drops permanently to ~1/3 of original (~700-800 FPS), regardless of settings or clean restarts.
This drop persists across Minecraft launches, system reboots, and no crash reports or logs are generated.
No unusual CPU, GPU, or RAM usage spikes. Other games and clients (e.g., Lunar) run at expected FPS.

Steps to Reproduce:
1. Install Simply Optimized
2. Launch the instance and verify high FPS.
3. Play for a few sessions. (Around 8 hours total)
4. Open the instance
6. Observe that FPS has dropped significantly and remains low permanently.

What I’ve Tried:
Ensuring all mods, Fabric loader, and dependencies are up to date
Running the game with default JVM arguments and memory settings
Duplicating the modpack
Testing on different launchers (Prism, Modrinth app)
Deleting instance cache
Confirmed issue persists without crashing, and no logs suggest errors

[cputnam-a11y]

Have you observed this in an instance without any mods installed?

[Kennxn]

I have not tried that yet, mainly because I don’t want to spend 8 hours playing an instance thats not optimized.