Post install
After you install the modpack, you may take a few extra things to squeeze every last bit of performance out of it. This section covers advanced topics most of the time so it may not be absolutely correct. If you find the information inaccurate, tell this to me in Discord! See the sidebar for navigation -->
If you're generally experiencing lag, try reducing your render distance. It's something everyone should know about, and it matters way more in singleplayer (so you might as well raise it a bit on multiplayer servers). Simulation distance only matters in singleplayer, and affects how far away from you things are still fully processed. 5 chunks is generally just fine, but you can raise it if you have CPU power to spare.
VSync should be disabled if you really want every last bit of performance - the way it works by default sacrifices performance to sync frames. Ideally, you should use Adaptive VSync, which you can either enable for Minecraft in your GPU's control panel or get using the Sodium Extra mod (though you might have issues with it on AMD GPUs)
You needn't put your graphics to anything above Fast, unless you're worried about transparent things rendering correctly at all times. Otherwise, almost the only thing it affects visually is whether leaves are transparent, which Sodium lets you control separately a bit later in the menu, so by setting Graphics to Fast and Leaves quality to Fancy you get fancy leaves for no performance loss from any of the less noticable changes in Fancy graphics.
Weather quality controls how far away from you weather effects are still rendered, it doesn't have much of a visual impact but putting it to fast can improve performance quite a bit whenever there's weather.
Particles don't have much of an impact on performance except for some nether biomes or simular areas, keep it at "All" unless you're experiencing particle-related performance issues on your configuration.
Smooth lighting and biome blending don't matter for FPS, only for chunk loading speeds. Chunk loading is really fast with Sodium anyway, so no need to worry about those.
Mipmap levels also don't matter for performance enough to lower them in most cases. The visual improvement they bring is well worth the small performance impact, so keep it at 3 or 4 unless you're specifically having issues.
"Chunk Update Threads" controls how many CPU threads are used to update changed and load new chunks. You should probably keep this at default, since raising it may cause framerate stability issues, but if you're using very high render distance you could try increasing it to make chunk loading faster.
"Always Defer Chunk Updates" makes chunk updates lazier, which may help a lot in cases with many chunk updates (huge piston contraptions or explosions) but may also cause important chunk updates not to happen, causing visual lag. Keep it enabled unless experiencing issues with broken blocks not disappearing and placed blocks not appearing.
There are multiple JVM distributions you may use which have additional optimizations Minecraft can benefit from. GraalVM is a good one, with even the cut-down CE edition a good boost in chunk loading and GUI rendering. If you are fine with creating an Oracle account you can download its EE Enterprise Edition which has more optimizations. The article linked below also has information on the subject (of GraalVM)
JVM flags are also a good way to improve the performance; good flags lie at https://github.com/brucethemoose/Minecraft-Java-Performance-Flags ; it's less of a guide and more of a list of flags to make the JVM do minecraft well. I'm not going to paste the flags from there here (as that's just a ton of text + they are constantly being refined) but in my rough testing they bring a good improvement over defaults. Just follow along and gather your flags; there is a different flag set if you are using GraalVM EE. Those flags can really help some people who are experiencing lag spikes.
Don't forget to set your -Xms*G and -Xmx*G flags to the amount of RAM you want to give to Minecraft! Setting them both to the same value is said to improve performance as well.
Don't use the original Aikar's flags for your server or client. Those are not relevant for modern Java versions.