after effects video playback/render

iMac 5k 27 late

4 GHz Intel Core i7

32 GB 1867 MHz DDR3

AMD Radeon R9 M395X 4096 MB

i have 15 layers with 45 seconds of video in total and when i play it it is very very slow. each layer has some effects and video. i am new at after effects, the tutorials are difficult without visuals. i have no idea what is rendering or if it is rendering or how to render (though it is suppose to do so itself and very very quickly). i was told i can use as how many layers i want to create a video with the specs i have. pretty frustrating for an expensive machine.

please help