whys my video play slowly?(after effects)

Try changing the video format.

Try exporting your video.

It is slow because it it live rendering the layers you have

When previewing video, it is generally slow especially if you import a video that is very large for it's length, (eg very little compression)

Certain formats are also more computationally expensive for after effects to render in a preview.

also if you have a multicore CPU, then enable the multiprocessing (it is disabled by default. it is good especially if you have a quad core or higher.

To directly answer the question: It's slow because the computer is not fast enough to merge all the layers, play all the effects, as well as the video. After Effects is not designed to 'watch' movies. It's intended for adding layers of effects using multiple compositions. All video must be rendered before it can be played at full speed.

However, there are a few solutions:

1. reduce the quality of the output (this is just your view, and doesn't effect the final output)
2. reduce the quality of the input (use a temporary smaller video file, just so you can get through the editing process faster, without waiting for After Effects as long as you're editing.
3. Increase the number of CPUs and/or amount of memory After Effects uses
4. Increase the amount of time for the "video preview"
5. Improve the hardware performance (disk drives, memory, CPU speed) of your computer

After effects exports so much information for use with a final project. Look at the file size. I imagine it's huge. I run it through either Final Cut Pro, Premiere Pro or Avid.