Lag is generally when the game looks hideously choppy. It's usually a phenomenon that occurs when you're overloaded with objects. Overloading with foliage, which can be very strong, can be avoided by making forests instead of individual trees, which is required when making jungles (Like the Colombia island I'm currently making, it'll be about 90% jungle when it's done). The same idea would apply to crop fields. Also, if you're using jungles (I know, I'm obsessed with jungles
), then you don't need to make your terrain rolling. It can be primitive and angular, but it'll look great when it's covered in forest. This technique is good anytime you want large collections of foliage in one area.
Some islands, especially jungle islands or islands with crop fields, need large amounts of objects to look good. When you're designing an island like this, you simply can't please people who are either too cheap to either improve their system or buy a new one. These are the people who instead bitch that "This island is too laggy on my 600MHz. Your island sucks, it's a piece of shit". You can see this charming type of response by looking through virtually any board about Tonal. Apparently, the whiners live in their own little world where everything must be dragged down to their level in order to be good.