I am guessing you are new to After Effects and do not understand the user interface and time display. If so please make sure your version of AE is up to date and then start here: Basic AE.
If that does not solve your problem try this:
- Select the composition you are having problems with in the Project Panel so we can see the composition settings
- Select the layers that are giving you problems in the timeline and press the U key twice to reveal all of the modified properties of those layers
- Make sure that the Timeline panel is showing you the entire timeline by using the small icons at the bottom left that look like mountains to expand the timeline view
- Press Print screen on your keyboard
- Return to this forum, add a reply and give us workflow details so we know exactly what you are trying to accomplish, then press the return key and paste your screenshot (Ctrl/Cmnd + v) so we can see what is going on
It's impossible to give you much help without details and a compressive screenshot.
Sorry for this issue. Did you ever find a solution? Please let us know if you have or if you still need help.
I'm guessing it's a caching issue. You probably need to let the frames cache before it will play smoothly and if your cache is full or maxed then it will always play slow (like a frame at a time). You may need to purge your cache or depending on whats in your scene, 16 gigs of RAM might only be enough to preview a few seconds in real time.
Also, what you probably don't realize is that most tutorials are edited. They edit out the part where they sit there & cache their frames for 10 minutes so that you don't have to sit there watching them sit there watching their frames cache for 10 minutes. It's sort of like cooking shows, when they put the raw pie in the oven and then immediately pull out a fully baked pie. "Hey...how come MY pie takes an hour and theirs was ready instantly??"
If you are new you will save yourself (and the folks who try to help you) alot of time by following a learning path designed for absolute beginners in order, starting with settings and preferences & moving on to even more settings and preferences before finally getting into more advanced settings and preferences.
If you go straight to tutorials that presume or expect you to have some fundamental knowledge & understanding of the program then you're very likely going to have problems. Not only that but because you skipped over the fundamental building blocks that teach you necessary concepts, you will also run into problems trying to get help with your problems. You might be able cook something up with a week old, cloudy eyed fish but your better off getting yourself a fishing rod and learning how to fish.