The mistake is in the first panel. I bet you won’t see it. I’m not referring to the false subject (there’s) either:
Here’s the goof: technically, “slower” is an adjective, and she’s using it as an adverb, to modify “drive.” You can have a slow driver, but you drive slowly, or in this case, more slowly.
Ah, idiomatic spoken English is so full of solecisms…