。
She's the fastest runner in the class.
Literal
She [topic-は] class [of-の] within [in-で] number-one foot [subject-が] fast.
足が速い (literally 'feet are fast') is the idiomatic way Japanese says 'is fast / a fast runner' — Japanese often locates a quality on a body part rather than the person as a whole. Similar examples: 頭がいい (smart, 'head is good'), 耳がいい (good ear), 口が悪い (foul-mouthed). 一番 ('number one') prefixed to an adjective makes it superlative, 'the fastest.' Note also: 足が速い uses 速い for 'fast in motion'; the homophone 足が早い (with 早) means 'spoils quickly' — a real distinction worth keeping straight.