This is simply how AE works. Only RAM previews guarantee realtime playback. AE is not an edit suite or realtime game engine, it's a compositing program.
Thanks for your reply.
When I use "space button" to preview, it will appear a green bar on the top of the panel. Is that means rendering the project? When the green bar done loading, the project is still not fluent.Is that normal?
My mac is 16GB ram.
Sorry for this dumm question.
RAM preview plays cached frames at real-time speed. Pressing spacebar does not.
Follow the link that I provided to learn the basics.