Let me give you some basics about working with still images in After Effects. First, a lot of newbies use still images that are way too big for the job. Video is pixels and scale of any pixel based asset should be at or near 100% in your project. I've seen many projects from my students where they were using still images that were never more than 20% to 25% of the size of the comp. Check your images. If they are more than twice the size of your composition you are wasting time and in the long run not getting the highest quality. If you are seeing scale values of more than about 125% your images are too small. Unless you have monster hardware and extremely fast fiber networks you should not expect to work on any projects that are larger than 4K, most projects should be 1920 X 1080 pixels unless you must produce 4K content. Size your images appropriately.
If the layers are 3D then you can't go by scale alone. You have to go by distance from the camera. When the distance between the camera and a 3D layer is equal to the zoom value of the layer the image is at the equivalent of 100% scale. If the image fills the frame and at 100% scale and you move the camera way back so the image only fills 10% of the frame you can then scale up the 3D layer until it fills the frame (1000%) and it will still be at an effective 100% scale and you can work with it efficiently.
Second, every frame must be rendered everything anything in After Effects is changed. This is a slow process. The proper technique for working with any compositing app is to work our any motion and composition previews at reduced resolution. I call these pencil tests. The easiest way to do this is to set the Magnification ratio, first option on the bottom left of the composition panel to 50% or if working with 4K 25% and set the resolution, the option somewhere in the middle, to auto. Work our your motion and composition and test the previews with those settings. When everything is the way you want it you start adding your effects, motion blur and other elements. If they look good at 50% magnification and auto (half) resolution then check a few critical frames at 100% (200% if you have a Retina or other High Resolution display) to make sure that the details are correct. Now send your project to render using a suitable delivery format and check the playback in a media player at full resolution. You can only judge video when it is moving at full resolution. No other checks or previews are 100% viable. You cannot reliably preview more than a few seconds at full resolution and full screen in AE unless you have a monster system and a hugely fast system. Even if you can, the motion may not be identical to the final rendered file because of many factors.
Last point. AE is designed for and good at creating shots and short sequences. It is horrible at editing movies. It was never designed to do that. If your AE comps contain more than a sentence or two of audio or last more than just a few seconds you are working inefficiently and making changes to tweak the project into perfection will take way to long. You have to learn how to work with shots to get what you want and then you should edit them in Premiere Pro into your final movie. More than 90% of my AE projects are under 10 seconds. Probably 75% are 6 seconds or less. Only on a rare occasion will I create a comp with more than one or two sentences of copy.