So I wonder if anyone can help explain this.
I have an early 2011 MacBook Pro 13". When operating normally and not attached to any external devices, the CPU runs at around 40-50C (all temps provided by iStat). When at my desk and attached to my external hard drives and USB hub (and a cooling pad), it runs at around 60C. Add my external monitor (a humble 17" Dell, connected via a MiniDisplay Port - DVI adapter cable), and that adds a few degrees, getting it up close to 70C (all temps measured after I'd been running in that configuration for at least ten minutes).
So in a general rearrangement of my desk a few days ago, I decided to try turning my monitor 90 degrees into portrait mode. From then on, my CPU started running at about 85C, with the fan running constantly at 4-5000RPM, and a noticeable general slowdown in the system (dragging windows around became stuttery, and so on). These all manifested almost immediately after boot - zero to broiling in a couple of minutes. The strange thing is, I saw no increase in CPU or RAM usage. My system just got hotter and slower.
I tried running with everything else but the monitor plugged in, and temps were normal. I turned the monitor back into landscape mode, and voila - my CPU temps and performance went back to normal.
So I'm back in landscape mode for the foreseeable future, but it still kind of bugs me that I have no explanation for this very bizarre-seeming issue. Can anyone else offer any explanation for it?