Iris (a native fabric alternative to Optifine) is quite a good alternative. I've started using it over Optifine exclusively. It supports almost all the same shaders that Optifine does without any support libraries needing to be installed, and it runs about 3-4x better than Optifine could ever dream of. I easily get 120+ FPS at 32 chunks with Sildurs' Vibrant Extreme Volumetric Lighting shaders with Iris.
There is a downside however, that Iris usually seems to cause a transparent entity glitch when run with Aristois. I have been able to fix the issue, but I can't honestly tell you how if you have it. I installed a few libraries for other client side mods and cleaned out all my MC installations and it seemed to have solved the issue for me.
You can install it as a Fabric mod the same way you install other Fabric mods with Aristois. Navigate to .minecraft/libraries/me/deftware/EMC-F-v2, select your version and pop the .jar files in there.
If you want to use an Optifine cape that you may have with Iris, there's a mod called 'Fabric Capes' (Edit: It used to be called Capes+ iirc, but seems to have been renamed.) that allows you to use it despite not running Optifine. It has a library dependency mod but I've had no conflicts yet.
Good luck!