Huge thanks to MeatballCraft modpack for their additional optimization guide.
On a personal note - make sure you really need Fugue and Scalar, as well as MixinBooter and ConfigAnytime (maybe you can safely remove / skip them).
Client / Single version = mods to increase FPS, speed up chunk loading, enhance control of game settings, and reduce memory and CPU usage, as well as optimize other logic (combined, since many mods are used both on the server and on the client. However, hosting services cannot work with Cleanroom Relauncher mod, so the Server version remains separate (as of now)).
A huge thanks to Opticraft for the incredibly helpful explanation of the current modded 1.12.2 situation.
Extremely recommend using Get It Together, Drops! if you're planning to dig a lot of blocks (ideally, live on a skyblock or in a void world, or something similar).
Some configs have been changed.
BlahajASM (especially: removeForgeSecurityManager, classCaching).
Forge itself (especially: alwaysSetupTerrainOffThread).
Some mods are from other sources, so they may not update automatically. It is recommended to check after downloading the modpack whether they have been updated:
Highly recommend these mods, not included because they don't align with Extreme Optimization! concept and "Base" 
/ there is a chance that their optimization has already been implemented by another mod(s) / other reasons, but may be useful:
!Don't forget to check the dependencies and notes, especially when using them with Cleanroom.
Sledgehammer - Smashes the stupid out of the client & server (has a "thin" version for Cleanroom) (actually, a really good mod, but requires meticulous configuration tuning to suit each user's needs).
Had Enough Items - Item and Recipe viewing mod for Minecraft with a focus on stability, performance, and ease of use (obviously, this is not just an optimization mod, but I can't leave out the fact that there's a JEI fork).
Relictium - Fork of Vintagium, which is a fork and backport of Sodium (decided to use Nothirium instead, but you can try).
Vintagium - Unofficial Fork and backport of CaffeineMC's "Sodium".
Valkyrie - Designed to optimize client-side operations while simultaneously introducing some valuable features.
Particle Culling - Stops rendering particles that the player cannot see (already fixed by other mods + a truly huge number of particles is required to start causing issues).
Entity Culling by tr7zw - Using async path-tracing to hide Block-/Entities that are not visible (outdated).
Entity Culling by meldexun - Client-side performance mod which aims to skip rendering of hidden (tile-)entities. (especially recommended for modpacks with massive dungeons with spawners).
PolyPatcher - Full of Vanilla bug fixes, Quality of Life improvements, and performance enhancements (too much that could negatively affect compatibility with optimization mods and huge modpacks in general, plus Universal Tweaks is already in use).
Born in a Barn - Fixes a Minecraft bug where Village doors keep chunks loaded (might already be fixed by other mods).
Fix Experience Bug - Fixes bug where experience does not display (and cannot be used) after changing dimensions (might already be fixed by other mods, or you might not encounter it at all).
Redirectionor [Retro] - Replacing references to element values in enum classes with static final field values references.
* "Dependencies" may not show all the mods.
| Name | Version | |
|---|---|---|
 | Alfheim | 1.5 |
 | BlahajASM | 5.28hf |
 | Cleanroom Relauncher | 0.2.2 |
 | MixinBooter | 10.6 |
 | Red Core | 0.6 |
 | Universal Tweaks | 1.15.0 |
 | VintageFix | 0.5.5 |
| Fixeroo-2.3.5-hotfix.1.jar | — | |
| naughthirium-2.2.1.jar | — | |
| Nothirium-1.12.2-0.4.5-beta.jar | — | |
| RenderLib-1.12.2-1.4.3.jar | — |
