。
How fast she's running!
Literal
She [topic-は] what fast running-is [explanatory-の] [I wonder/exclamation-でしょう].
速く is the adverbial of 速い ('fast') — i-adjectives drop い and add く to form their adverbial. 走っている is the present progressive 'is running' (activity verb + ~ている); the polite-spoken/literary でしょう gives the whole exclamation a slightly observational, narrating tone — as if the speaker is watching her run by and remarking on it.