10-20% damage increase? What? I HIGHLY recommend you read up on how stat increases work. Even a full set of 6* glyphs is unlikely to give you a 10-20% damage increase.
I mean Fleatoad is definitely wrong about the low rank glyphs but a full set of 6* glyphs can absolutely give a 20% damage increase if not more. First, if it's an ATK champ, that's potentially 32% Bonus ATK score before crit dmg multipliers THEN you're potentially talking 40 points of Speed (5x8) which is a increase to total turns.
Not to mention Rank 6 Accuracy Glyphs can roll a ton, so much so that you can replace an ACC Banner with a ATK Banner or something which is even MORE damage.
Rank 6 Glyphs are huge differencemakers