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It should be. Indeed, cavalry should be even more powerful than in ancient times, thanks sturrups
Also, your vikingr are ill-armoured, and riders in reality strike from higher position in melee, horses (especially moving) are far more hard to kill than average infantryman. Do not forget, vikingrs are basic LIGHT infantry, they are meat shield.
Use archers to weaken cavalry from distance, use spearmen to engage in close sombat, surround enemy cavalry unit, immobilise it, you'll see a difference.