Major Campaign Lag on Co-op

My friend and I were trying to complete the campaign on Legendary together, and we were experiencing heavy lag throughout it. I was kicked out of the game twice because of it. It’s really starting to get on my nerves.

Both me and my friend have open NATs, good connection, and we don’t lag anywhere else aside from campaign. What gives?

Anyone know of a solution?

i encountered the same thing during certain points of my campaign with a friend. it was heaviest during large battles/ outside area’s, and minimal inside areas… tried many different things, router flush, modem flush, all the tech things… nothing fixed it… just had to tolerate it as it bad net code written into halo 4… was also bad, playing system link did not fix this issue…