How come rendering a 10th of my composition takes half the time it takes to render it fully?

[It_s_Pizza]

I had a small issue somewhere on a render, so I'm using the area of interest to render this specific section again and paste it onto my final footage. It's about 1/10 of my full composition, yet it takes 1h30 to render while my full composition took 4h !

How does that even makes sense ? On top of that, it's a corner area where the animation going on is very light in terms of performance required. I really don't get it, do you know a few technics to re render a specific section without absurd render times ?

[Dave_LaRonde]

Well, since we know ABSOLUTELY NOTHING about effects used, expressions used,  footage used, 3D / 2D, lights, etc....

No, we have no clue what to tell you. It could very well be you're doing all you can. 

[Rick Gerard · Accepted solution]

4 hours could be normal. I've had comps that took four or five minutes a frame to render. My design limit is about 7 minutes a frame for complex things. I have also created a lot of comps that render at around five or ten frames a second. A couple of months ago I created a comp that was just a couple dozen frames but it had about 40 layers so the composite worked. It took about a half-hour to render those 40 frames. Render time always depends on what you are doing.

Generally, it is a very bad idea to try and use AE to render a movie. Most of my comps are a single shot and under seven seconds because I use AE only to create composites and motion graphics that I cannot create in an NLE. I've done films that are eighty to ninety percent effects shots but those were all edited in an NLE like Premiere Pro after all of the effects shots were created as individual comps in AE and rendered as footage to use in the final project. On a very rare occasion, I will combine a dozen or so comps into a master comp that is a few minutes long but that only happens with extremely simple comps that I am sure are going to render very quickly.

Describe your project in detail, give us the workflow and if possible a screenshot of the comp where the slowdown happens that shows all of the modified properties of the layers in that section (select them, then press uu) so we have some idea about what is going on. My first guess is that you are just not using AE the way it was designed and you are trying to do way too much in a single comp.

[P.M.B]

As Rick said, it depends on what going on in those frames.  If those frames are very cpu intensive they will take longer to render.