I actually had no idea it was faster solo than in a party. I know enemies scale, but given previous instances of scalable enemies I expected it would still be faster with multiple people.

They really missed a trick with the rewards for this though. They could have tied Relic Armour to Variant/Criterion dungeons.
Variant drops coins that you use to buy Sil'dihn armour, Criterion drops items you use to give the armour a special effect like a Haste within Sil'dihn, which can then be used in Criterion Savage.
This would have made all modes relevant, Variant for glam, Criterion for Savage prep, and Savage because it's Savage.