We can assume that a larger snail moves faster simply by nature of being bigger. If we assume the snail's movement speed scales linearly with its size we can estimate its speed.
In the image he is twice as tall as the buildings which appear to be about 4 stories tall. 14 feet per story, times 8 = 112 feet. A snail is about 1.5 inches tall.
112 feet is 1344 inches. Divided by snail height, this snail appears to be about 900 times larger than normal.
Humans sprint at about 14 mph. A normal snail moves at 0.03 mph. Multiply that by 900 and our giant snail clocks in at 27 mph! Better find a car if you don't want to drown in snail mucus.
