You have not said how you tune the two needles, but to me it seems like like you have set it too lean on the low speed needle (the midrange needle would actually be a better name as it effects the whole middle range and can even restrict the full throttle setting). A video would help with the diagnosis though.

Set the main needle on the rich side, so that it drops a little in the peak rpm. Then set the low end on the rich side too. If the low speed needle is set to what may seem "just right" at constant rpm then you will get a slow throttle response. It needs to be set on the rich side of that seemingly correct setting, so that you get crisp throttle response. I.e. you set the low end needle to get a crisp throttle response rather than doing a pinch test or something else at a constant rpm.