Fair enough. Where's it said that it slows Karna down though?
Achilles isn't on his chariot. This is the battle against Vlad in the hallways of the hanging gardens. It's mentioning that even if Achilles isn't on his mount, he's still the fastest hero. And this specifically refers to him batting away Vlad's stakes faster than Karna did.I am pretty sure that being on chariot doesn't give Achilles better reflexes so from that context it is pretty clear what kind of speed the book is talking about.