I have been getting many questions recently from people asking me why their renders are not playing back smoothly in Quicktime. Now for people who have been doing this for a long time, the answer to this may be obvious, however I have found there are many people who don't know what is going on.
The reason for the issue is the use of the Animation Codec. By default in After Effects, when you choose to export as a Quicktime, animation is automatically chosen. Those who are not familiar with After Effects and compression leave the default settings and render out.
Now, Animation is a great codec. When the quality is set to 100%, it is lossless and supports an alpha channel. The problem is that there is so much data that most computers (even higher end ones) are not able to decode all the information fast enough to playback in real time. So even though the playback looks choppy, it isn't. There is nothing wrong with your render, your computer just can't keep up. If you brought your render back into After Effects and did a little RAM preview, everything would be perfectly smooth.
So what is Animation for? It's what I would call an intermediate codec. It can be lossless, so it's great for going between programs without losing any quality. For example, if you finished a motion graphics sequence in After Effects and were moving back to Avid or Final Cut to add it to your project before doing a final render. Using animation in this case is great because it makes sure you have the highest quality image before doing a final render. (There are many more times when using Animation can be useful, this is just one example)
Now you might be asking, "Well what should I use instead?". There are many options and it really depends on where your render is going. If you are going to the web, you probably want to render out using H264. You still get the high resolution, but your video is compressed and able to play back normally. If you need a high quality render that plays back better then Animation, try Apple ProRes 422 or ProRes 4444. Neither of these codecs are lossless, however they can produce very high-quality video. The fourth four in 4444 is for the aplpha channel which means that you can even store transparency data with that codec if needed.
Also, one last thing you should know. After Effects in not compression software. Even though it has the ability to export to many different formats, it will not be able to produce the same quality as a program like Compressor or Squeeze, or even Adobe Media Encoder. If you have access to any of these programs, I suggest rendering very high quality out of After Effects, (maybe using Animation) and then do your final render in a program that is made for it.
Long blog today. Sorry about that. Hope it made at least a little sense. You should know this is a very brief explanation of compression. I recommend you search around to find out more. It will help you deliver higher quality content.