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I'm assuming that your first renders are using the lossless or uncompressed preset. There are very few systems that will playback lossless video in real time because the data rate exceedes the maximum transfer rate from a hard drive. It's as simple as that. Lossless codecs are not designed for playback, they are designed to allow you to have a source file that is identical in every way to the original.
If you are rendering to h.264 you should be using the Adobe Media Encoder because AE cannot do multi pass rendering from AE's render modules. H.264 is a delivery codec designed to deliver up streaming video so it's data rate is well within the limits of most systems. That's why it plays back smoothly. That is what it was designed to do.
Thanks for the links to the FAQ. That was very helpful.