Modded Minecraft often needs to use more ram than Vanilla does to ensure it runs smoothly.
If you’re having performance issues, or your game crashes with an OutOfMemoryException, then you should try allocate more ram in your launcher settings.
It’s a good idea to be conservative with the amount of ram you allocate. You want to assign enough to allow your game to load and run without the OutOfMemoryException crashes, but not so much that Java struggles to manage the memory (causing frame-stuttering/lag).
A good starting point for modded minecraft is 2GB.
Follow the steps taken in the video to open the launch options for your modpack profile.
Under ‘JVM Args’ modify the -Xmx#G value, changing the # to the number of Gigabytes you want to assign.
This will become the maximum amount of ram that the game can use.
Open the launcher Options window and modify the ‘Maximum memory’ value.
Note - this value is in Megabytes, not Gigabytes.
There are 1024 Megabytes in 1 Gigabyte, so to allocate 2 Gigabytes of ram, enter a value of 2048.