OptiFlex helps to maximize the performance of device's memory. It automatically keeps frequently-used apps in the memory as long as possible to prevent longer waiting times each time the apps reload. The more available memory there is, the more apps it can keep.
You can find OptiFlex from Settings > Advanced. The apps kept in memory are chosen automatically by the system. You can also manually set it up for apps that are already in use.
Note: The operation steps/information in this FAQ may not be applicable to all types/series of Zenfone product. Some options in the screenshots and operating procedures may slightly vary, depending on the software and firmware version.