Under advanced options in the loader editor, set these java options.

Be sure to update the path to where your forge installation is located.

-Xmx6128m -Xms3256m -XX:PermSize=1256m -Dminecraft.applet.TargetDirectory="C:\Games\FTB Presents Direwolf20 1.12.2 2.5.0" -Dfml.ignorePatchDiscrepancies=true -Dfml.ignoreInvalidMinecraftCertificates=true -Duser.language=en -Duser.country=US