Online forge and custom games

so I’ve always had really bad internet and it is very rare to be able to play any online games (including the halo games) so with the older halo games I just messed around on forge and custom games with with a bro on splitscreen but now that splitscreen is gone in halo 5 I can’t play with my bros and to make matters worse both forge and custom games are online so I can’t even play alone. Could you guys tell me why they are both online and if there’s any way I could At least cut down on a little bit of the lag. Thanks