Making Unity Run Better/Faster?

I've been using Unity for about a year now and I understand the basics, but do not know the details of how Unity works. Are there ways to make it run faster, such as by changing how things render or the resolution in edit mode. I'm using a brand new MacBook Pro, and Unity severely lags when I use the brush to place down large groups of trees or bushes, and when I raise or lower the terrain.

Any advice would be greatly appreciated. Thanks!