We have no way of knowing why it takes so long.
We don't know the effects you are using. We don't know if you have 3D layers. We don't know if you have multiple lights, which slow things down. We don't know if you are trying to use ray-tracing, which takes a REALLY long time.
However, we DO know that you're trying to make an h.264 file, which takes a bit longer and is also not good. You should make your AE animations in a media container in a lossless codec. Using it,. you can transcode to any kind of delivery specification.
I use Paint Bucket (stamp) to erease 40 to 50 small (5x5pixels) tracking points. I used in anther comp trapcode particular with 200 particules/sec with no particular movements (no physics). This comp is inserted in the main comp with a polar coordinate (that's the watermark, and the film is mapped in a sphere, so polar coordinate is usefull to recreate flat images at the bottom of the sphere.
I'm actually creating VR films, unfortunately there is just the h.264 (.mp4) codec to be compatible with Samsung Gear VR Headset.
With Lossless codec i'll have a big big big movie..
I think it's the paint bucket that's slowing down rendering. But i don't understand why first frames are calculated in 20 seconds, and why later, similar frames with same effects are now calculated in 45 sec. To illustrate this i join screenshots.