How to serve a large vector dataset on GeoServer

• I have a GeoServer on a VPS (6 cores, 24GB RAM).
• I created PostGIS store with a dataset of around 12GB (4.4 M polygons).
• From store I imported 10 layers (each represents a region of a country). It takes 2-3 seconds to load a WMS tile on average from these layers.
• I want to serve all of these layers as a single service, but when I import all of them into a single Layer Group, it takes more than 60 seconds to render a single WMS tile, so I receive a TIMEOUT error.

Example:

Loading a WMS tile from layer A (bbox 0-10) takes 2 seconds.
Loading a WMS tile from layer B (bbox 10-20) takes 2 seconds.
...
Loading a WMS tile from layer J (bbox 90-100) takes 2 seconds.
Loading a WMS tile from Layer Group A-J (bbox 0-100) takes 60+ seconds. (Even if this tile is within layer A bbox only).

I expected this Layer Group to load a tile in 4s~ max. There should be some sort of major performance loss that I don’t know about. What is the bottleneck here? Maybe I should use different technology for this?