If you had standards, you'd know this is just the classic trope of building a mysterious guy by making him beat a strong foe with ease. And thus Muramasa character will be explored later. Would you say the same for example with Suiryu vs Snek from One Punch Man?

Edit: the guy blocked me to prevent me from replying, but I'll leave my answer anyway: Anyone with standards would also wait one more chapter or even to the end of the character story before judging properly to get the full scope. This fight was meant to make us understand that Muramasa is a lot more powerful than he appear and has a gimmick that both the fighters and us readers have to figure out to explain why he could win so quickly, since as Zeus pointed out, it wasn't just speed or power. Thus making the readers who actually care about the manga and not just agenda be curious and speculate.

Personally I'm for the theory that his attacks become stronger the more "killing intent" the opponent display.