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
fasted
an cheeda ride
It's a metaphor
You could say - a fast cheetah or a fast dolphin
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lighting travels at the speed of light
Chris's mind is a fast-moving river.
No. If you're not sure if a comparison is a metaphor or a simile, just check and see if it uses the words "like" or "as". If it does, then it's a simile. This is a simile.