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.
Fortunately, snail speed doesn‘t scale linearly with size, as the relativ slowing down of oxygen diffusion into the cells would shut down metabolic processes in the snail and lead to its asphyxiation.
I remember reading once that the human skeleton couldn't possibly support a 50-foot giant. Or even a 10-foot giant. The world's tallest people have all kinds of skeletal issues. The tallest man in recorded history, Robert Wadlow 8 ft 11.1 in (2.72 m), had to wear leg braces because his leg bones were so brittle. Also, he couldn't feel anything in his lower legs and feet because his nerves down there just didn't have the energy to make it to his brain.
His name is Deep Roy and he actually has quite the number of IMBD credits. Lots of suit work like Warwick Davis. In fact, both of them were in Return of the Jedi. Warwick Davis was Wikkit the Ewok and Deep Roy was Droopy Mccool, the big pink guy who played some sort of horn in the Max Reebo band.