Always at the peak of travel (max height) ANS

At the peak, there is only one component of velocity and that is horizontal only. At all other points alone its path, the object has both THIS (same size) horizontal velocity plus a vertical component. The Σ of these two components always being > the horizontal one alone.

The velocity is smallest after the ball hits the ground and stops, duh!

...

the velocity of the projectile is the vector sum of the horizontal and vertical components of velocity

the horizontal v is essentially constant

but

the vertical v is zero at the top of the arc (it is going neither up nor down at that point)

so

the vector sum is minimum at this point.

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