rendering gots slower and slower... (PAIN!)

The video is showing my issue and its very annoying because it is not possible to render a video like this to the end, just when i wait 5 hours or so for 4 seconds. The weird thing is when i render the same video which is 4 minutes long then its rendering the first 20 seconds very fast and when the video is just 6 seconds it just renders the first 2 seconds fast and gets slower. Seems like it is nearly exponential getting slower... so it is impossible to render like this. I need quick help! It is the same for every project btw. Can you tell me what pc specs I should give you for this problem?