From the article:
Q: If I add a lot of images on my course pages, will it affect performance?
A: Yes, it will. The number of reads/writes to the database skyrockets when you add images in labels. A page with 100 images requires over 3 times the amount of calls to the database compared with a page with no images in labels. Looking at iMacros loading pages during the tests, I also noticed that images inserted as URL’s (i.e. not uploaded to Moodle) show up much faster in Moodle pages than the images uploaded to Moodle.
That's really interesting. Seems that 2.x moodle's approach to dumping images into the database as blobs is really hard on servers compared to 1.9.x's simply storing them in the moodledata directory.
Again the big problem i think for most people is how much CPU/SQL is used per call. It limits how many students can put in requests AT THE EXACT same time. Moodle can handle a million students as long as they don't all push a button or login within 2 seconds of each other.
I really wish that Moodle had a queueing architecture where requests could be queued rather than requsting everything processing