You will not be able to work with a particle system efficiently if you are not able to control your particle count. There's multiple solutions at your disposal.

one of the most obvious, is hiding the particles you are not working with by pressing on the ''computer screen'' icon. This button will hide your particle system only in the viewport, not in the final render. This button is located per particles row inside your main particle selection area in the tweaking panel, or in the creation panel where a statistic feature is available to quickly control what's displayed and what's not.

If you desire to hide all your terrain particles at once (if the computer is freezing because of massive particle-count for example) search for the "hide all target particles" operator.

The second solution is the "Hide Percentage" property. This property will hide n amount of particles depending on the percentage value, only in your 3D viewport.

You can have a clear indication of the particle-count showing up on viewport or on render just under this property.

Note that this technique is not as effective, 1% of 1 million particles may still hurt your performance a bit more compared to having 100% of 100k particles.

Next to the property there's a rendered icon button, if toggled to True, all percentages will be automatically on 100% on rendered view, then reset back to original values when the viewport render is back to solid.

When creating your particle system by using the preset or the biome workflow, the created system may be either hidden by default, as you have the ability to hide particles or reduce particles percentage on creation (see creation>creation behavior>universal options).

Or they may be hidden automatically by automatic security actions, Scatter will try to protect you from crashes when you create a particle system with too many particles (see creation>security actions). More about security actions in Scattering behaviors.

Ideally, we'd need to choose assets with details adapted to our scenes perspectives, but we all know mosts assets available on the market for ray traced sofwares are way too high poly, so will be our job to deal with the consequent slowdowns.

Natively, blender only solution to optimize an asset in the viewport is the bounding box display option. You can reach it natively in blender "Properties " > "Object" > "Viewport Display " > "Display as" or quickly toggle it in Scatter "Tweaking " > "Instances " > "bounding box Button".

A 'Proxy' in the digital world means a lot of things, in the archviz/environmental art industry it mainly refer to a simple concept: replace a high-poly mesh by a custom low-poly mesh, only in the viewport, and use the full resolution asset in the final render.

Most 3D packages disposes of such system but not Blender, so we decided to created "Proxify" that quickly evolved into "Lodify", a free universal Level of detail addon, already adopted by addons such as Graswald or shipped directly with products such as Terrascape.

Note that blender kinda dispose of a 'proxy' system already, newly renamed 'library override', but it's a specific system meant to be used for teams in animation workflow for rigging and animating a character. Not quite the tool environmental and archviz artists are expecting.

Thanks to Jakub uhlik, an experimental point cloud generation display option is available in the tweaking panel.

This point-cloud visualization technique is extremely customizable and will for sure optimize your viewport if you are working with meshes composed from millions of polygons.

if you are interested in this technology, check out Point Cloud Visualizer on the blender market.