Yellow attacks are normalized, which mean they ignore the speed of the weapon. But the damage your attack does still does a roll of your weapons damage range for its calculation, so the slower weapon should have a higher damage range and do more damage. Its just not going to be as big of a deal.
The big thing about BGs is it takes the speed of the weapon and applies it own templated damage range for that particular bracket. That's why 4.0 speed grays that normally do (1-2 damage) are so huge. Template takes 4.0 speed and assess a huge damage range for that speed and ignores what the weapon says. Now if you are wondering if this has been nerfed I'm not sure... that's how it worked in 7.0.