Minecraft is a brick game created by Notch in 2009, which would later evolve from being a one man project, into being run by a small set of people that shorty after became Mojang.

Known Issues

1.21.5+ has degraded performance

According to Sodium's official description for Sodium 1.21.7, 1.21.5 seems to have degraded performance, and while not unplayable, it may be lower than users are expecting. There is not much documentation on this yet, but in the meantime, you should downgrade to 1.21.4 if you can.

1.21.5 has also been skipped by many performance mods, most notably OptiFine.

Incompatibilities

ViaFabricPlus

ViaFabricPlus seems to have known issues with C2ME, as it's the first mod you are told to disable on it's official Wiki.

Other PSAs

OptiFine is not recommended

OptiFine is an aging mod. Most of it's benefits have been done better and stronger by current mods such as Sodium (and it's many forks) and Angelica. Using OptiFine in a scenario where Sodium is available is just not worth it. There's many reasons why you shouldn't such as OptiFine themselves having skipped version 1.21.5 (which was an update so bad, even Sodium warned you that their mod might be ineffective), OptiFine relying on outdated enhancements that were last effective in 2012, poor mod incompatibility issues, and even just OptiFine itself having a few sketchy moments that might be documented later on in the future.

If you are on a 1.15+ enviornment, just use Sodium. 1.7 users have access to Angelica, and while every other version in-between might not have anything 100% perfect yet, Legacy Fabric is still making its rounds as more people give it a spin. There's no point using a mod that's gone ineffective and mostly replaced, as several people warn about.

Further reading

• TheUsefulList's warning, which might be easier to understand for some beginners
• This QuiltMC post by Emmaffle also has some good reading and some mod recommendations if you use other parts of OptiFine.

Some other notes

• sp allegedly declined an offer to get OptiFine baked into Minecraft very early on in Minecraft's release (1.5 era forum post).

Be careful what flags you add

Some Java arguments might be snake oil and may do more harm than good. More information can be found here.

General notes

Resources

• Sodium's wiki page on Supported Launchers might provide some good ideas on some launchers that are generally favored by the community.
• TheUsefulList's General Minecraft resources, such as Launchers and Server manager software.
• If you used Lunar client and broke away recently, try this list for alternative mods that add Lunar client's old functions!