ModernFix is the successor to ArchaicFix, and is designed to fix bugs and improve the performance of Forge for Minecraft 1.16.5. Currently the focus is on reducing startup and world load time, however other optimizations and bugfixes will likely be added in the future.
As of v1.3.0, ModernFix moves JEI loading to its own thread so that it does not slow down world loading.
The (admittedly limited) group of testers so far have seen their existing worlds open 2-3x faster from the main menu with ModernFix installed. Nonetheless, please be sure to make a backup before trying it out.
If you encounter any crashes with ModernFix installed, please open an issue on GitHub.
For even more performance, I highly recommend the following mods: