If speed does not make the game itself faster it could at least slow down everything else, so so the character will take about the same amount of time to dish out the dmg to pass through the level but from slimes perspective it was a blitz.
Speed seems non concessional, as a quick example: sword attack lvl:70 SPD:1321 interval:0.62 , after rebirth SPD:18 interval:0.65 . Here goes my fantasies about a speed devi- angel. At least show modifier values in the System>>>Stats of by how much things get multiplied by, as to not get any false hopes up.
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