I think the high amount of lag in this game is caused by how much is going on at any given time. Bullets are simulated instead of hitscan (which is great. Please don't make it hitscan), the 3 different suit modes, visor tags, the possibility of the suit-arms lock while ADS, footstep tracking (for the module), and so on.
I'm not going to assume I know anything about how this game is coded, but here's some suggestions I think would help:
1. Make the shotguns' pellets disappear after a set distance. I hate to suggest following Black Ops lead on anything to do with shotguns, but the min damage these have is so low that it might as well be 0, so just make it so. Thats 7/8 less bullets to track.
2. Take away some of the suit abilities that are massively underused, such as the lock arms feature
3. I really like tagging and how it could be useful, but maybe it, too?