Hello !
Client is lagged when he walk slowly, host work good.
My problem:
Slow walk when aim BLUEPRINT:
Normal and Slow is replicated and run on server.
Please help me 
Does this only occur when aiming down sights? This probably isn’t a final solution, but it might be the case that the client is predicting a faster movement speed than what the speed actually is, as Max Walk Speed does not appear to be replicated. Try this, when the client aims, set the value on the server, and locally. See if that fixes it, at least temporarily.
If I were you, I’d switch it from tick, to only whenever aim is changed. There is no reason to set it if the value will be the same.
This post is a year old but I thought Id add what I found. I was similarly setting the max walk speed on my character and found that I had severe lag issues as the client. I fixed this by leaving the max walk speed set to a high value and changing the input axis instead. The network seemed to have a really hard time calculating the movement positions when max walk speed is changing so I would recommend you use a different method of changing speed such as I did by changing the axis value from the input.
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indeed