The complexity of the map doesn't affect the lag of online servers. It can however decrease your framerate if you're using an older computer to play. If this is the case, I recommend altering your graphics settings for the game. M&B conveniently displays an estimated performance percentage based on the graphics options you select. I tend to keep mine at about 150% or so. The options I most recommend are turning off anti-aliasing and HDR lighting. Also make tree detail low, and use fast reflections for water. This will make a minimal impact on the appearance of the game and save your GPU a whole lot of trouble.
