I have an 8gb i5 7200u nvidia 920mx laptop.I want to optimize aftereffects because preview mode is so slow and not showing realtime fps.My composition is 1080p

Okay, good luck

Get more ram, AE is not really usable with 8gb, even the recommended 16gb is painful, get atleast 32gb

@ItsNotMyFault I would throw any software to the trash can that requires at least 32 gigs ram and burn that trash can to make sure no one uses it.

It turns out it's because of frame rate preview .When I downgrade it to 8 fps it works smoothly.Strange I thought I had a decent graphic card.

@hack video editing is a bit special, raw(uncompressed) high resolution video data takes up massive amounts of memory (AMD has gpus with a few TB of SSD storage built in for this kind of work since I/O quickly becomes a bottleneck otherwise, 32gb is pretty much minimum if you work with 4k video.

24fps at 1080p uncompressed required ~3Gbps If your ssd can't deliver that to the gpu for encoding then you need to be able keep the part you're working with in ram/vram (~1GB for every 3 seconds of raw video you want to have buffered)