You’re asking for real-time rendering or better. The only way to get improve rendering speed would be to reduce the comp window resolution and quality, use low-res proxies, switch off effects and maybe use OpenGL, but that’s dodgy.
AE doesn’t respond like a video app — it has to render every frame. This depends mostly on the processor, not the hard drive. Even if you do nothing to a comp that contains one piece of footage, AE still needs to render every frame, not like a video editing app.
Of course, once you do a RAM preview and cache the frames, you can scrub in realtime, but once you change something, you have to do a RAM preview and cache those frames again.