What causes long delay after hitting run in editor, how to reduce it?

I click the run button, which immediately darkens; then nothing more happens for at least 8 seconds, then it finally switches from "Scene" view to "Play" view and the game starts. Is this normal / expected behavior for Unity?

It didn't just suddenly start doing this, but I don't know when it did start or how gradually. I tried disabling all the scripts active at scene start, which reduced the time slightly but not much. But a new, empty scene will start in <2 seconds. I just checked AngryBots and it takes maybe 5 seconds for the audio to start playing, then game on in about 7. There's a lot more going on in the Angry Bots scene than there is in my scene.

8 seconds doesn't seem like long to wait- but it really adds up when I'm trying to test many incremental changes in a script and I'm already frustrated by the amount of lag in MonoDevelop. Can anyone tell me what is the Unity editor doing in this time; and is there a way to make it do less of it?

Or is my problem just that I have a 5-year old Macbook Pro and need to upgrade? It's 2.4Ghz with 2GB RAM and still seems fast enough for my purposes the rest of the time.