This is a "Base Slider Velocity", it affects the whole sliders in that specific difficulty. If it sets a higher value, the slider goes faster.
If you want that specific slider to be fast, head over to "Timing Setup Panel" and click on the "green plus button" - it will say Add Timing Section as stated in the picture.
Be sure that "Inherit previous timing settings" is ticked and see that a small green circle is shown beside the timing section.
The next picture shows "Slider Velocity Multiplier" - where the inherited timing section that you added will be multiplied with the "Base Slider Velocity".