What does each speed reading exercise practise?
Each exercise makes a different reading behaviour visible. Meaning groups focus on links between words, main-idea practice on the centre of a passage, and scanning on locating a defined fact. Visual tasks are controlled attention warm-ups, not vision therapy.
Which exercise should come first?
Start with the reading purpose. Use main idea or chunking when meaning is fragile, scanning when looking for a defined fact, and controlled line tracking when you often lose your place.
When should the level be reduced?
Reduce pace or difficulty if comprehension falls, discomfort appears or the task turns into guessing. A larger group, faster display or shorter time is not success by itself.