Viper Strike of the Mamba Subsequent Hits While holding the key are weaker

I put together a demonstration with my pob on the shaper first stage.
https://youtu.be/WVUH3bKGPwU

Subsequent hits while holding mamba seem to be significantly weaker.

I have hits can't be evaded on both daggers and 100% poison chance.
My poison duration is .3s here so each attack should be poisoning.

Theoretically hitting faster would also apply more wither stacks and result in a faster kill times but it is slower in reality.