Ok a guidline for un laggy map is a total file size of less than 50kb.
Keep the amount of trees to a minimum.
Ive always prided myself on making maps that dont lag,check any of my mindset snipers maps from the last four years and u will see what i mean.
Also you can further reduce screen lag by proper placement,on the med i only place objects on complete flat ground.That means everything,trees,houses,the whole nine yards.
Go to
http://www.mindsetsnipers.com and d/l and examine any of my maps iam sure it will offer some inspiration into smooth map making.