Well, the boat anchor saga seems to be coming to a close. I decided to revisit the issue with Dell support. As it turns out, the Inspiron XPS Gen 1 (and, I assume, future generations) has a feature wherein performance is automagically degraded as temperature increases. Apparently this is some sort of self-perservation mechanism. Yes, I clean the fan vents a few weeks ago.
It turns out that this machine has heat sinks on the fans' output side. So the support representative this time asked if I cleaned inside the fan. I had not. It turns out that the fans are module and simply pop-out/-in very easily. So, under his direction, I remove the fans and WOW!!!!!!!!!!! It was FILTHY in there. There was virtually no airflow. Pretty disgusting actually.
I don't know why the first Dell representative didn't suggest it. I am, however, grateful that things seem to be on the mend with my workstation. However, it's cool that I have a rebuilt, clean system. Things are running smoothly (knock on wood). So, keep your systems and your socks clean...especially your systems.