Why bother?
Once upon a time, OptiFine was the way, to make your game better, faster, and stronger. Things have since changed.
Fabric/Quilt
NOTE: Most Fabric mods require the Fabric API mod in order to work, while most Quilt mods require Quilted Fabric API to work.
Sodium
Sodium is the best client-side optimisation mod available at the moment, and we highly recommend installing it when possible.
To install in PolyMC, create an instance with the Fabric (or Quilt) loader, and then go to the Mods tab and click Download Mods. There, click on Sodium, click Select mod for download, and then press OK.
Iris
Iris allows you to use OptiFine shaderpacks, while also running Sodium. It works well, but doesn't yet support every shader.
You can find a list of currently supported shaderpacks here.
To install in PolyMC, create an instance with the Fabric (or Quilt) loader, and then go to the Mods tab and click Download Mods. There, click on Iris, click Select mod for download, and then press OK.
Other Mods
If you want to further improve your performance, please also consider installing Lithium and Starlight!
If you would like to go a bit further, LambdAurora maintains a very detailed list of OptiFine alternatives for Fabric and Quilt, which you can find here.
Fabulously Optimized
If you don't want to search, and install these mods manually, then try the Fabulously Optimized modpack.
See the MultiMC/MultiMC (auto-update) guide for help installing it. These instructions will work for PolyMC too.
Forge
Note about Forge
Unlike on Fabric/Quilt, most unofficial ports of OptiFine alternatives for Forge might have some compatibility/stability issues.
While on some modpacks/with some mods they're going to work good, with others they might be very unstable so be well aware of what you're doing. If you have issues, use OptiFine.
Don't report issues with those on upstream's Discord support channels!
Rubidium
Rubidium is a Sodium port for Forge. Be aware of the statements above before using it.
To install in PolyMC, create an instance with the Forge loader, and then go to the Mods tab, click Download Mods. There, search for Rubidium, click on it, click Select mod for download, and then press OK.
Oculus
Oculus is an Iris port for Forge. Be aware of the statements above before using it.
To install in PolyMC, create an instance with the Forge loader, and then go to the Mods tab, click Download Mods. There, search for Oculus, click on it, click Select mod for download, and then press OK.
Other mods
You can find a list of performance mods for Forge here.