Coroutines waiting for random times?

shiftDown = false
momentum = 0
x = game:GetService("UserInputService")
x.InputBegan:connect(function(key)
    if key.KeyCode == Enum.KeyCode.LeftShift then
        shiftDown = true
        print("test")
        newThread2 = coroutine.create(function()
            repeat
                if momentum == 0 then
                    momentum = 1
                end
                if momentum == 1 then
                    humanoid.WalkSpeed = 20
                    momentum = 2
                elseif momentum == 2 then
                    humanoid.WalkSpeed = 32
                end
                wait(3)
            until shiftDown == false
        end)
        coroutine.resume(newThread2)
    end
end)
x.InputEnded:connect(function(key)
    if key.KeyCode == Enum.KeyCode.LeftShift then
        shiftDown = false
        wait()
        humanoid.WalkSpeed = 10
        momentum = 0
        print("Player stopped running from momentum "..momentum)
    end
end)

This script works fine, mostly. When you hold down shift for the first time, your walkspeed goes from 10 to 20. After 3 seconds, it goes from 20 to 32. This is as expected. Now, when you release shift and hold it down again, your walkspeed goes from 10 to 20 as expected... and then instead of waiting 3 seconds, it seems to wait a random amount of time before boostin your WalkSpeed to 32. I believe this is to do with how coroutines work, but I'm not entirely sure how they do.