What is WindowServer? In brief, it handles windows. It is the core process responsible for drawing the windows and boxes that make up the graphical user interface. As : "The window server does more than just manage windows. Even an application with no user interface (like a background-only application) depends on the window server."

In other words, it's important. And it's exactly the sort of process that, if it got bogged down, would produce the sort of symptom I was seeing.

I now had a prime suspect. But the key question remained unanswered: Why was WindowServer bogging down? It had never done so in Snow Leopard. And it wasn't causing problems on my other Macs with Lion installed. So what made my Mac Pro running Lion special?

I assumed there was yet another piece of software involved. Most likely, it was something from my Login Items list, something not on my other Macs. These items launch automatically at startup, allowing them to cause trouble without the user having to take any "manual" action. But which item?

I searched online for possible guidance. I found several forum threads dealing with WindowServer slowdowns, describing symptoms similar to my own. But the threads either came to no conclusion or identified a cause that was not relevant to my particulars. Dead end.