As you might already know the multiplayer in this game is very laggy.

I figured out a little solution that makes it less exaggerated.

So, to fix this you need to increase the bandwidth by changing the variable in config:
1. Open config file {Serious Sam 2 install folder}\Content\SeriousSam2\Sam2.ini.
2. Search for cli_iMaxBPS under Engine section. Set it to 65536, 64kb/s (by default it's set to 6000, If i'm not mistaken). You could try even bigger value, like 524288, i didn't notice any change.
3. Save config.
4. In main menu check variable value. Press `~` type cli_iMaxBPS, press Enter. It should display value from config file that you've set.
5. Join to some game and give it a try.

In my case, it made enemies to not lag at all. But there is still lags when you jumping on uneven ground, or standing on moving platform.

Oh and alternatively, you can set variable through console in main menu (before joining in). Press `~` and type cli_iMaxBPS = 65536, and press Enter.