This issue has been ongoing for over 4 years now. When this game launched in the broken state that it was in, I was still able to play H2A/H4 because I was able to aim correctly with no slow-turn. After an update to the game a little less than a year after the game came out, the aiming for H2A/H4 became completely broken for me on the Legacy Thumbstick Layout setting.
Now for those of you who don’t know what Legacy Thumbstick Layout is, it’s just a different way to aim. I’ve been aiming this way since Halo 1. Default Thumbstick Layout players have joysticks that separate aiming and moving. They aim with the bottom right joystick and move with the top left joystick. Legacy Thumbstick Layout players aim and move with both combined.
This picture illustrates what I mean: https://i.imgur.com/WWfrlBx.jpg
The problem continues to exist when you use “Modern Aiming” in this new update that 343 just released. If you looked at the previous picture, the top left joystick can move forward AND look left and right. If a Legacy player orients the top left joystick in a diagonal position (left or right in four 45 degree variances: top-left, top-right, bottom-left, bottom-right) the player will be looking left OR right and moving forward OR backwards at the same time depending on which diagonal direction the player chose. When this occurs with this new update, an INSANE amount of slow-turn occurs rendering this new “Modern” aiming setting completely unusable for ANY Legacy player on ANY Halo games on MCC.
This video demonstrates the problem: Halo MCC "Modern Aiming" Slow-Turn on Legacy Thumbstick Layout - YouTube (please forgive the white noise, no clue why that’s happening)
With the “Classic” aiming setting, this does not happen and it’s what I have been playing with all this time (although it wasn’t a visible setting). Although it’s usable, the aiming doesn’t feel as good as it used to when I played Halo 3 on the Xbox 360 with Legacy Thumbstick Layout. I am curious to see whether or not the new “Modern” aiming setting will allow me to get closer to that Halo 3 Xbox 360 style of aiming that was so good years ago. Also, please do not tell me to just learn how to play on Default Thumbstick Layout, I have been playing Halo since Halo 1 and Legacy Thumbstick Layout, as I mentioned previously, is deeply embedded into me. I have already tried learning Default Thumbstick Layout back in Halo: Reach and after 4 or 5 weeks, I gave up because I could not get used to it.
Please 343, I ask you to investigate this issue and see what’s wrong because I would like to aim better too 
Cheers!