I can only speculate, but I would guess that the techniques she was taught in the speed reading class were less efficient than whatever she was already doing without thinking about it, so she regressed towards the average speed for a person with speed reading training, which was lower than where she started. She said that her reading speed decreased noticeably while reading similar text in similar situations, although of course it's possible that she was experiencing selective perception.

One possible explanation is simply awareness.

If you naturally develop a technique, you may not be consciously aware of it at all. But take some training, and all of a sudden your conscious brain is butting in going "this is the way to do it".

And, well, your CPU is going to be less efficient than a well-optimised RPU (Reading Processing Unit)