No, it's due to using the "then" weirdly to introduce the main sentence after the conditional adverbial clause. It sounds weird in English - arguably wrong in this case - but would be perfectly grammatical in German. Better:

"If it doesn't help, you're not driving fast enough"

If you insist on using the "then", you'd have to put it at the end of the main clause:

"If it doesn't help, you're not driving fast enough [then].

More obvious in the comma-free structure:

"You're not driving fast enough then if it doesn't help."

Whereas in German, the "Wenn" allows for a "dann" (that literally translates to "then"):

"Wenn es nicht hilft, [dann] fährst du nicht schnell genug."