"But also Mars....same gravity !! So , impactors are possibly faster on Mercury than the Moon ."

Mercury's escape velocity is closer to Mars than the Moon, but it's orbital speed is a far more significant factor as far the velocities of impactors.

An impactor velocity is mostly dependent upon the planet speed and the difference of inclination of the body which hitting the planet. And the orbital speed of the asteroid [or comet].
Some equatorial escape velocities [km/sec]:
Earth: 11.18
Moon: 2.38
Mars: 5.02
Mercury: 4.25
Jupiter: 59.56

Mercury's orbital velocity is 47.88 km/sec.
Earth's orbital velocity is 29.79 km/sec

Most impactors which are asteroids hitting Earth will be around and over 20 km/sec and roughly comets or the parts of them hitting earth will be around 50 km/sec.
With Mercury it's around twice these velocities.
So it's quite possible that some objects hitting Mercury could exceed the velocity of any objects which hit Jupiter.

In terms of minimum velocity possible, this would be indicated by the escape velocity of the planet- so no object will hit Jupiter below, say, 59 km/sec.
And with Mercury the minimum velocity possible is 4.25 km/sec