Co Op Latency and input lag

Tried playing the Campaigns with my brother on online co op as we live in different countries but we encountered input lag on the team member who wasn’t the Fire team leader. The delay is around a second long and effects everything but your right thumb stick looking control.

I Found this is from Halo support:

"Co-Op Latency: At this time, we have not been able to identify any issues around this feature and the functionality is on par with the rest of MCC. There is no active development happening around improving/re-writing the legacy network code around this area of the game. Unfortunately, this would be a massive undertaking and just isn’t within the realm of what the team can currently support."

Can someone explain why it would be such a big undertaking? I just want to play with my brother like in the ol’ days