>>1024901
in general, less meshes is better, less materials is better.

But some modifiers like subdiv or mesh deform or corrective smooth or physics are very expensive, so you should minimize the number of vertices affected. Which means not joining things that don't need to be if they use one of those, and putting modifiers that generate new vertices as far to the end of the modifier stack as possible.

Also if animating a character is unexpectedly sluggish, look at its subdiv modifier. if it says it can't run on GPU (which happens depending on what modifiers are before it) set the viewport subdiv level to 0 or use the simplify setting for the scene.

If you have multiple similar objects just make one and put it in a collection, then use collection instances for the others, it'll speed things up massively.

If shaders are too slow (try in material preview and in solid view and look at the difference) stay in solid view, but you can bake an object texture to use in solid mode.

Disable collections you don't need. Like, not hide them from render or viewport but disable them, the tickmark on a given collection. This way they won't take up any processing time apart from objects inside that are used by objects in active collections.