Light important sampling (LIS) optimizes rendering by spending more computing time on the light sources that matter most.
Without LIS, you must manually specify how many samples the renderer is to take for each area light in the scene. But with LIS, the renderer determines this automatically and dynamically, so lights that are closer and brighter receive more samples. With LIS, it is not necessary to set the number of area light samples on the lights themselves; instead, you use the LIS Quality setting to increase the number of light samples globally.
Enabling LIS does not provide an immediate improvement in performance because the number of light samples will be exactly the same as if LIS were off. But because those samples are better distributed, quality should improve greatly. When you convert an existing scene to LIS, start with a lowered LIS Quality: Try 0.25 or 0.5; this should provide similar quality at greatly improved performance.
LIS will provide most benefits in scenes with a large number of lights—especially area lights. Scenes with a small number of lights might not benefit from LIS. Scenes with a single light will not benefit from LIS.
Because of the algorithms it uses, LIS works only with physically-plausible light sources: That is, lights that fall off at a realistic inverse-square rate. LIS does not consider lights that fall off at other rates, so it provides no benefit for such lights.