My question is why there is 'slower' instead of 'slowly'?

Hi teachers,
I know the sentence given is correct. My question is why there is 'slower' instead of 'more slowly'?
Isn't 'slower' a comparative adjective?
Deborah drives a bit slower and much more carefully than him.
Another question please, could 'inferior velocity' be a suitable explanation for 'slower'?

Thanks in advance.