video loop slow down

I this is a video and not a composition then create a new composition from the video by dragging it to the new composition icon in the project panel, move the CTI to the last frame of the composition, which should correspond to the last frame of the video, and observe the frame number. If you can't see the frame number then Ctrl/Cmnd click on the time indicator in the time line. Now open up the composition settings and precisely double the frame count. If your original comp was 1291 frames then the new length would be 2582 frames.

Now apply Time Remapping to the video layer. This will give you a time keyframe at the first frame and just past the last keyframe of the video. If you move the CTI to the keyframe by pressing the K key the comp panel will be empty (black). This is normal because the keyframe is inserted after the video has stopped playing. Here's the trick. Move back one frame (Ctrl/Cmnd + left arrow or Page UP) and set a new time keyframe for time remapping by pressing the diamond shaped icon in the timeline next to the keyframe indicator.. The value for that keyframe should be the same as the original comp length. Now delete the last time keyframe, move the CTI to the end of the comp (frame (2582 in my example) and then hold down the shift key and drag the last keyframe to the CTI and it will snap in place. The value of the keyframe will not change. All that is left to do is to reset the out point of the layer to the last frame of the comp. You can do that by pressing Alt/Option + ] or by dragging.

Choose the most pleasing method of frame blending and render the new video. If the graphic loop has a transparent background then use the Lossless With Alpha preset that comes with the Output module in AE to render your new movie.