We often upload meshes and forget how much complexity they had when we did it, and we often buy or get freebies while hypergridding without thinking about what the cost is on our server because each mesh prim has a land impact of 1 no matter how complex it is. HOWEVER, complexity still impacts us, because it puts a load on our viewer to render the mesh. The more vertexes, triangles, etc there are in the sim, the more load it puts on your viewer, so creators want to minimize the complexity of a build in the application they create it in, and or pick a lower level of detail on the mesh upload interface. However we don't often remember what those are unless we document everything, and have no idea what the complexity is of others work.
BUT, there are tools on the viewer that help you measure this, which are found in the Developer menu.
If you are unaware of this menu, or it is not visible, go to Preferences, to the Advanced tab, and check "Show Developer Menu".
The Developer menu will then appear at the top of your viewer. Then go to the Developer menu and select "Show Info", then from that submenu, select "Show render info". A data display will appear in the lower left corner of your viewer with a lot of data in it about all sorts of rendering thats going on. What is important is down near the bottom, the third to last line. If you don't have any objects or link prims selected, it will show the total number of triangles and vertexes of all objects in the sim, how much memory those objects consume, and how many objects there are.
However if you have one object or prim selected, that same line will show you just the data for your selection. This is how you can measure the complexity of an extant object in sim, even one you did not upload yourself.