need tips on how to make a large terrain run more quickly and not lag when scrolling in and out.

I will typically clip the terrain to the area I absolutely need and make sure I thin the points as much as possible. Then I will create a 2D drawing where I merge in the contours. And a 3D drawing with the terrain model and calculated features turned off. That way I can use the 2D model for drainage evaluation, etc. and the 3D model for modeling purposes. The 2D model with contours merged in still slows performance some but not as much.

How do I ...."make sure I thin the points as much as possible"

This is a good reference.

 https://www.youtube.com/watch?v=Gy17m0ac_kc&ab_channel=BentleyOpenRoads 

Basically make sure you only have ground points on.

There is also other methods such as export the point cloud (you can import .laz files and MicroStation will convert them to a .pod) as a .las file this will thin the point cloud and then you can make the terrain from the .las and also use the filer option in the import terrain.

Typically, POD files are very taxing.  If this is a raw LAS data source, my recommendation is to change the density of the POD down as low as 10 (or to what seems reasonable), clip the POD down to smaller reasonable areas if needed, create terrain from smaller POD and export your POD produced Terrain out to DTM, TIN or LandXML then import that file (Terrain import) back in for any future terrain editing and/or use by OpenRoads technology. 

Typically, POD files are very taxing.  If this is a raw LAS data source, my recommendation is to change the density of the POD down as low as 10 (or to what seems reasonable), clip the POD down to smaller reasonable areas if needed, create terrain from smaller POD and export your POD produced Terrain out to DTM, TIN or LandXML then import that file (Terrain import) back in for any future terrain editing and/or use by OpenRoads technology.