Rendering in Adobe Media Encoder gets gradually slow down

When I am rendering my project through Adobe Media Encoder, it is very fast at the beginning but gradually slows down later on. I opened and checked the output folder. It produces frames very fast during the first 2~3 minutes. Almost 20~30 frames per a minute. However, it makes barely 2~3 frames only in the end. Why is this happening? It doesn't matter which project I render. It happens to all projects. Even though all frames have basically same quality and effects. (My projects are not even complicated or heavy.) I googled it already but it seems like no one has this issue. Does anybody know why it happens and the solution?