Slow motion in preview

Check file interpretation first by selecting the footage in the Project panel and choosing File>Interpret Footage>Main (Alt/Option + Ctrl/Cmnd + g). Make sure the frame rate is what it is supposed to be. When I shoot 59.94 with my most common camera it is interpreted as 29.97 because that is how I set up the metadata. That means one second of real-time is going to take 2 seconds to playback. It doesn't make any difference what the comp or sequence frame rate is. Playback speed depends entirely on the frame rate that your editing app (AE or PPro or any other) decides to assign to the footage. That number is not always accurate.

If the footage is properly interpreted then the slow playback and dropped frames are happening because your system can't keep up with the current preview settings. Try changing them. My first check for playback is to look at the Info panel when running a preview, My first adjustment to give me real-time preview is to lower the Comp panel Magnification ratio and make sure that Comp resolution is set to Auto. If that does not help I set the Preview Panel to skip frames. Most experienced AE artists don't try and look for anything but timing when they run previews in AE. If the motion is good and a few critical frames are checked for accuracy with motion blur and all effects turned on then it's time to render and move on to the next project.

Without any system info and exact details about the footage used, preview settings etc. nobody can really even begin to guess.

Mylenium