Blender setting problem

Hi all,
what’s the best blender setting?
When I have some high poly object and do some video the animation preview is running very slow.
I had a look on performance and my hardware usage is very low (example - CPU: 20%, GPU: 2%, RAM: 34%) and the video is so slow that it is running on about 2fps.
Why?

Any ideas please.

Thanks.

My hardware:
CPU: AMD RYZEN 1700x
MB: Gigabyte ax370 gaming 5 (rev.1, BIOS v. F8)
GC: AMD Radeon r9 270 2GB + shared
RAM: 8 GB (bit to low I know)
OS: Windows 10 Pro 64