Can someone explain why adobe after effect is so slow and then this problem occurred?

[ellahunter1]

[Rick Gerard]

Composition settings, available RAM, and effects all can contribute to the problem. Without knowing workflow and comp details as well as system and OS specifications it's pretty hard to narrow down the cause. Start digging by soloing layers and turning off effects. If the comp settings are not at fault, then it's probably a layer with an effect that is causing your problem.