Dave Filoni had a perfect opportunity to recapture the lightning fast demon 2003 Grevous was, with pristine CGI technology, but instead he gave us the most generic mustache twirling villian-of-the-week imaginable.

Grevious had to be all fucked up and slow with a cough in episode 3 because Mace smashed his organs like a Pepsi can in Genndy Wars. That makes sense.

Filoni's Clone Wars on the other hand takes place before Grievous captured the Chancellor, so him being fucked up and slow and coughing *already* doesn't make sense.