Talk:Help/Optimizing Performance/@comment-26065993-20150212074947/@comment-25612015-20150322092706

Use the "JVM Arguments" setting of your Forge profile.

Personnaly, i have no problems with "-Xmn1G -Xmx4G" (between 1 to 4 Gb), but my Minecraft client was crawling and often crashed with the default settings (between 128M to 1G).