This sentence would not be considered ah metaphor. It is a simile because it uses te word 'as' to compare to unlike things (galloping and fences have nothing to do with lightning). A metaphor does not use the words 'like' or 'as' when comparing.
As fast as a cheetah. All the athlete runners are fast cheetahs.
a metaphor for fast
He she is flash
Fast as flash speedy as lightning
lighting
fasted
lighting travels at the speed of light
15
It's a metaphor
an cheeda ride
You could say - a fast cheetah or a fast dolphin
Pertty fast like fast as lighting speed