Why does pre-rendered footage get re-rendered again for every playback or preview?

Question

Dear Sir/Mdm,
I'm a novice, please bear with me.
I'm using an Asus laptop with:
     1. CPU: Intel i5-3337U @ 1.80 GHz (4 CPUs)
     2. OS: Windows 8.1 64 bits (6.3, Build 9600)
     3. RAM: 8 GB
     4. DirectX: Version 11
     5. On-board Graphics: Intel HD 4000
     6. Dedicated Graphics:  GeForce GT 740M
     7. Dedicated video memory: 2048 MB DDR3
     8. Nvidia Driver version: 6.14.13.3523
     9. Total available graphics memory: 4096 MB
I hoped to avoid repeated rendering of footage segments during RAM previews, scrub-through, or playbacks.
I pre-composed related layers of completed segments and pre-rendered the pre-comp, which gave a footage (in AVI format) to replace the pre-comp.
But when I scrub-through, playback (spacebar), or do RAM previews, the pre-rendered footage gets re-rendered again (for every scrub-through/preview/playback).
Need advice.
Thanks.

Answer 1

Thanks for the responses and apologies for the slow reply (I spent some time following-up on the clues from the kind responses and did a more focused reading of AE's Help page based on the clues).
To answer Mr Szalam's questions:
     I'm using AE CS6 version 11.0.4.2
     Memory settings:
          Installed RAM: 7.9 GB
          RAM Reserved for other applications: 2 GB
          RAM Available for AE: 5.9 GB
          [Not selected] Reduce cache size when system is low on memory
     Previous Multiprocessing settings:
          Installed CPUs: 4
          CPUs reserved for other applications: 1
          RAM allocation per background CPU: 1.5 GB
          Actual CPUs that will be used: 0
     Current Multiprocessor settings: [Not selected] (due to advice given in the responses)
Copy that on the RAM part. Time for a little hardware upgrade :-)
For Mr/Mdm bogiesan's advice, thanks. Now I know more of what to look out for in simplifying future projects. For this current case, the pre-dendered movie-layer is the only layer-segment as I intended it to be a completed segment in the hope of avoiding repeated renders ... alas, it didn't turn out the way I hoped ... oh well :-)
For Mr Dave La Ronde's advice, copy that. I've disabled multiprocessing. Indeed, I'm using sub-par hardware for the intended purposes.
To sum up:
From AE's Help-pages:
1.  "If you have a nested composition ... you can save time during each rendering operation by pre-rendering the nested composition ..."
     Takeaway: Re-rendering will occur under all situations, be it a plain movie-layer, effects-applied movie-layer, etc. Pre-rendering was designed to save time on nested compositions only (i.e. re-rendering nested compositions take longer than re-rendering the pre-rendered version of the nested composition). 
2. "The rendering of a movie is the frame-by-frame rendering of each of the frames that make up the movie.".
     Takeaway: Even plain-movies get re-rendered. Since pre-rendered movies are the same as imported movies, everything gets re-rendered without exception.
Although Premiere Pro CS6 seems to handle movie footages with transition-effects more smoothly (e.g. during scrub-through), but I guess AE CS6 has much more to keep track of if it needs to tell the difference between a plain-movie and an effects-applied one ... perhaps a more optimized approach is indeed to render everything without exception. Perhaps a dedicated no-render-layer-slot can help AE to decide what not to render (i.e. anything dragged into such a no-render-layer will never be rendered) ... but that's just from a novice hobbyist's limited-knowledge perspective (I'm not a professional effects artist, which may be obvious from the sub-par hardware I use ... I bought the Teachers and Students Edition of CS6, which was kindly offered by Adobe at a steep discount ... and I use it to satisfy my curiosity of how those stuff-of-dreams sort of movie-effects are created ... for now, I'm only able to play with basic video montages :-)
Thanks for all the responses. Case closed as AE seems to be designed to render everything without exception.