If the surface on which a snail is moving is reduced by half, it would generally have the effect of increasing the snail's speed.
Snails move by propelling themselves forward through the rhythmic muscular contractions of their foot against the surface. When the surface is reduced, the snail's foot has to cover a smaller distance to traverse the same relative distance, resulting in shorter strides.
Assuming the snail maintains a consistent rhythm and frequency of muscular contractions, the reduced surface would allow the snail's foot to make contact with each new section of the surface more quickly. Consequently, the snail would take more steps within a given time frame compared to when the surface was larger.
This increased number of steps within a given time period would result in a higher overall speed for the snail. However, it's important to note that the absolute speed increase would depend on various factors such as the snail's individual stride length, frequency of contractions, and any potential constraints imposed by its own physiology.