How do i make AE run smoother?

The standard workflow, even with a very powerful system starts with setting the Comp resolution to Auto and previewing most of your work at a 50% magnification Ratio. It also involves turning off things like motion blur and some effects when you are checking timing and animation. The biggest productivity killer that I know is trying to use After Effects as a video editing app. Most of your comps should be one-shot that cannot be created in an NLE. 

Even on my most powerful workstation I always emulate the "Pencil Test" then "Ink and Paint" workflow that traditional animators developed to be able to get anything done. Here's the breakdown:

The Pencil Test

Design any motion or animation in the scene and work out the timing with the lowest resolution, simplest effects that you can. use to figure out how the scene works. If this was character animation, you would use pencil sketches to generate the pose to pose timing. If you are adding some spectacular particle and glow effects to a scene that needs a 3D character added, start with a very few numbers of particles and simple effects so you can get the placement and timing accurate. If the shot requires masking or Rotoscope, then, by all means, check the critical frames at 100% or more and verify the matte is accurate, but don't waste time previewing again and again at full resolution, just check the critical frames, then move on to the next step. For a lot of this work Fast Draft and even skipping a frame or even two in the preview does not hurt the process at all. When the framing, blocking, timing, and staging of the shot or sort scene is OK it is time to go to the next step.

Ink and Paint

This is where you turn on motion blur, turn on Final Quality, finalize the effects settings, get the particles and glows, and start doing the final compositing with light wrap, and color correction, and everything else that you need to make the shot perfect. When you are doing Ink and Paint it is hard to avoid the temptation to constantly run ram previews, but the efficient workflow is to just check the Hero frames. What is a Hero frame? That's a frame where everything must be working. When a few critical or hero frames have been checked, it's time to render. There is almost never anything to be gained by a tiny tweak and a full rez ram preview, then another tiny tweak and a ram preview. You really won't know until the project is rendered anyway so when Ink and Paint are complete, it is time to render. That is the only way you can be sure that the shot is working.

If you have a heavy production load I strongly recommend that you pick up a background rendering tool so you can move on to the next shot or scene while the production master DI and/or a preview for client approval is rendering. I use Render Garden. I own 3 of them but I prefer Render Garden because AE doesn't suffer any noticeable slowdown, especially when working on Pencil Tests.

That's how you work efficiently. My iMac Pro is in the shop for service and my MacBook Pro is on loan to a friend that is working on a different phase of the same project but does not have all the required tools on his system. I'm working on a 4K project on a backup Mac Mini that I use as a data server in my office to manage backups and archival storage. I'm still productive on this 2012 machine with only 16GB of ram because I am spending most of my time in Fast Draft and 1/4 resolution working on the timing for a very difficult 35-second scene that combines 3D character animation with an actor shot on a green screen stage with a moving camera that needs to be inserted in a street scene. Camera tracking, rotoscope, 3D character animation, compositing, light wrap, color correction, matching lighting, and a long sequence can easily be handled on this very old and weak system over this weekend because I am not hung up on ram previews and full-resolution comp panels and the half-minute sequence has been broken down into 9 comps that will be rendered and edited in Premiere Pro for the final scene. I do have to wait for some renders because the Mac Mini doesn't have any available cores for Render Garden to use, but that gives me time to comment on this thread.

Gotta go, time to move on the shot 4 in this sequence. It's 71 frames long and it will probably take me about an hour.

As @byroncortez mentioned its recommend to know your full pc spec to give you the correct advice. 

But for now in general am giving you the idea to make AE run smoother.

1. Buy an SSD (Solid-state Drive )  even 150 GB drive (If you're in Budget else buy 1TB SSD ) do the job you need to change your AE disk space to SSD

2. you told that you have 16 GB ram, its good for AE as per official Adobe system specification, but you have to change your Ram allocation for AE at least  14 GB ram  

3. Change the resolution type to half or quarter so you can preview it without lag.

4. Followed by step 1 you need to allocate more space for adobe cache.

5. don't use the OS folder to export your video, basically, it will make your CPU or GPU to do the heavy job in do both read-write task.

6. I Don't  have an idea what processor you use but using some i5 processor with some Nvidea graphic card also do the job easily 

7. if you have not update either windows or your graphic driver also you may face some lag even crash . so it recommed to update your graphic card always  (Adobe strongly recommends updating to NVIDIA driver 430.86 or later when using After Effects. Drivers prior to this have a known issue which can lead to a crash.)

Ok but give us more detailed information about your hardware and software, not just the RAM, but also the footage you are working with.