Big Mom actively blocked Kong Gun with haki, Jinbei actually hit her. That's why the attack actually did something. And why assume Jinbei's attack is weaker? He is a pretty strong character himself and a former shichibukai. Do you think Kong Gun is stronger than the strongest attack Mihawk has? I don't.
Brook didn't speed blitz Big Mom, he blitzed Zeus, and remember Brook has already shown he is fast and strong enough to cut those homies. Plus according to the writer Brook is the fastest Straw Hat, not Luffy, Zoro, or Sanji, Brook.
Finally remember Big Mom is not trying to kill the Straw Hats or sink the boat, she wants the cake above all else.
Doflamingo also attempted to block Kong Gun with armament haki and got sent flying. So unless Jinbei is capable of doing something like that to Doflamingo, it's only logical to assume that Kong Gun is stronger than Jinbei supposed strongest attack. Why even compare someone who fought with Shanks and is the world strongest swordsman, to a Fishman who is potentially the third or second strongest in his crew? And regardless Mihawk did attempt to attack a Yonkou with one of his strongest attacks only for that to be dodged by Jozu a commander.
Her and Zeus weren't able to react in time to Brook and thus were speed blitzed. Although that could be explained by the fact that Big Mom was taken back by the panties comments making this one less egregious than Jinbei and Nami bs.
She's barely conscious so I doubt she cares whether she kills them or not. According to some people Jinbei lost an arm during the fight which makes the chapter somewhat better, but I don't see how you can justify what happened in this chapter.