Expensive

In game design, an expensive entity/effect/function/whatever is one that takes an unusually long time to process at runtime. As an example, in Source, expensive-to-draw entities include water and dynamic light sources.

Care must be taken when implementing expensive concepts, and if possible, they should be avoided or optimized. On the other hand, if you have an area that is otherwise very cheap to draw, you have enough leeway to add something more expensive.