Why is our map so laggy when it has many players in a server?

The maps I’m speaking of are our Conscripted D-day and Vietnam Maps. We have gotten multiple complaints of lag on our maps. From our experience, it seems that whenever they get to the 30-40 player range(40 players is max for the server) it starts lagging and the ping glitches out saying it’s at 0. It’s definitely something related to the amount of players, as our map doesn’t lag when player counts are in the 10-30 range or so. I’ve seen complaints on lag with many players on a map in other forums. Is this really just a server issue that we have no control over or is there something with our map causing lag with higher player servers? We never had this issue before so something changed at some point. That’s why I don’t think it’s us. It would suck if we have no control of this, but it seems like that is the case unfortunately. If anyone could please help, it would be much appreciated