user23237: I've just installed after effects and I would like to learn it. 
When I click the play button, it seems like it starts to render so the frame rate is very low ( one frame per many seconds) and the the whole interface is laggy.
After rendering few frames, I don't remember what I really did but I clicked the play button again and the video was played smoothly. 
I clicked the play button again and it became laggy one more time.
I think the preview process should be different from the rendering process in After Effects (just like 3D max or any other similar software. There is a 3D view port that doesn't render the exact materials and lighting). The two processes are separated to make the play/pauses fast. 
I would like to know how do people use after effects with this mixed (Rendering & Preview) processes? Of course, I know that there is a method to get around it and I'm missing it. I would appreciate it if you give it to me.  

user1950: You've answered your own question; when you preview in After Effects it renders first, then plays back. The default setting is that if you hit numpad-0 to initiate a preview it will render the frames it can until it either fills up the available RAM, gets to the end of the preview area (work area or end of comp), or you hit numpad-0 again, at which time it will play back from RAM, theoretically much faster.
In recent iterations of the software, the playback performance has got significantly worse. I don't know why AE can't easily play back even UHD previews from RAM or even from disc at real time or faster, just like every other free media player app can. You get better playback performance if you have a comp dynamically linked in Premiere, it makes no sense. Unfortunately that's just the way it is, and the only thing you can do about it is learn Nuke, Fusion or Blender. 
Some tips for better real-time playback: updating the UI seems to take an inordinate amount of processing power. So having things like the graph editor, Lumetri scopes or even just the timeline visible while previewing will slow it down. I work on two monitors, and if I can't get real-time playback I will maximise (~) a window that doesn't get refreshed during playback, like the tools window, so that it fills the entire monitor, and that improves performance dramatically. Also making sure the comp window is zoomed to 100% will help, and if that doesn't work, lowering the preview resolution ctrl / ⌘shiftj to half or quarter ctrl / ⌘shiftaltj will speed things up.

23237: <p>I've just installed after effects and I would like to learn it. </p>

<p>When I click the play button, it seems like it starts to render so the frame rate is very low ( one frame per many seconds) and the the whole interface is laggy.</p>

<p>After rendering few frames, I don't remember what I really did but I clicked the play button again and the video was played smoothly. </p>

<p>I clicked the play button again and it became laggy one more time.</p>

<p>I think the preview process should be different from the rendering process in After Effects (just like 3D max or any other similar software. There is a 3D view port that doesn't render the exact materials and lighting). The two processes are separated to make the play/pauses fast. </p>

<p>I would like to know how do people use after effects with this mixed (Rendering &amp; Preview) processes? Of course, I know that there is a method to get around it and I'm missing it. I would appreciate it if you give it to me.  </p>
 1950: <p>You've answered your own question; when you preview in After Effects it renders first, then plays back. The default setting is that if you hit <kbd>numpad-0</kbd> to initiate a preview it will render the frames it can until it either fills up the available RAM, gets to the end of the preview area (work area or end of comp), or you hit <kbd>numpad-0</kbd> again, at which time it will play back from RAM, theoretically much faster.</p>

<p>In recent iterations of the software, the playback performance has got significantly <em>worse</em>. I don't know why AE can't easily play back even UHD previews from RAM or even from disc at real time or faster, just like every other free media player app can. You get better playback performance if you have a comp dynamically linked in Premiere, it makes no sense. Unfortunately that's just the way it is, and the only thing you can do about it is learn Nuke, Fusion or Blender. </p>

<p>Some tips for better real-time playback: updating the UI seems to take an inordinate amount of processing power. So having things like the graph editor, Lumetri scopes or even just the timeline visible while previewing will slow it down. I work on two monitors, and if I can't get real-time playback I will maximise (<kbd>~</kbd>) a window that doesn't get refreshed during playback, like the tools window, so that it fills the entire monitor, and that improves performance dramatically. Also making sure the comp window is zoomed to 100% will help, and if that doesn't work, lowering the preview resolution <kbd>ctrl / ⌘</kbd><kbd>shift</kbd><kbd>j</kbd> to half or quarter <kbd>ctrl / ⌘</kbd><kbd>shift</kbd><kbd>alt</kbd><kbd>j</kbd> will speed things up.</p>
