蛇 and 亀: speed and sequence of movement
Distinguish a particular observed scene from a general trait in snake and turtle examples.
A snake's movement
蛇, read へび, means snake. 蛇が草の間を進んでいました。 says that a snake was moving through the grass.
蛇を遠くから見ました。 means viewing a snake from a distance. It does not include approaching it.
A turtle's movement
亀, read かめ, means turtle or tortoise. 亀がゆっくり歩いていました。 says that one was walking slowly.
亀が水に入りました。 says that a turtle entered the water. Translating it as leaving the water reverses the direction.
A scene versus a generalization
この亀は先ほどより速く歩いています。 says that this turtle is walking faster than a little earlier. The comparison is with its earlier state, not another animal.
蛇が通った後で、亀が動き始めました。 says that the turtle began moving after the snake passed. It gives sequence without explicitly establishing cause.
What is the comparison point in 先ほどより速く?
Show the answer and why
The state a little earlier. It does not mean faster than another animal.