Once inertia is overcome and the object is in constant motion then sliding friction will mostly depend on the surface, environmental factors (wind, ice, etc...).
example:
To push a piano from 0 to 1m/minute you would need to over come a huge force pulling down the piano(gravity) but if it were already moving...to push it , it would require less force, if the environment (and the surface) was smooth and not uphill.
