Monster is right. When making a game, you are making trade-offs and balancing. You need to use tools and measure to find and fix problems efficiently. Some things are obvious but sometimes the worst offenders are hidden. There's good info available on how to use profilers, frame debuggers, etc.
Monster's "performance triangle" is a very good analogy. Here's a relevant example: The way in which you unwrap a model can actually affect the final vertex count as well as UV splits will count as extra verts: in the case of all those tree leaves, the vert counts between your two methods would be the same, since you're…