TLDR. This will reduce RAM usage, improve loading times and minimally reduce CPU usage. If you play on a pc with 12GB's of RAM or less and you're not using this, you're not doing it right. Even for users with 12+ GB's of RAM, it's free real estate, go get it!
- Head over to AdoptOpenJDK and select 'OpenJDK 8 (LTS)' and 'OpenJ9'.
- Download and run the installer.
- Accept the terms of the license agreement and click 'Next'.
- Unselect the 'Add to PATH' and 'Associate .jar' options. (You only need 'JDK with Eclipse OpenJ9') Then click 'Next'.
- Click 'Install' and finish installing.
- Open your 'MultiMC' and open the 'Settings'
- In the list to the left navigate to 'Java'.
- In the 'Java Runtime' section find 'Java path:' and click the navigation box.
- Navigate to 'C:/Program Files/AdoptOpenJDK/jdk-8.0.292.10-openj9/jre/bin/'.
- Find the 'javaw.exe' and double-click it.
- Excellent work. Your MultiMC will now launch using the new JVM, enjoy.