Use an index card as your own portable shutter. Like commercial gadgets, the card prevents you from regressing to previous lines of print. Also, because you use your arm and hand to move the card down the page, you are physically more focused on the reading. Do not forget to use soft focus and shortened margins as you read. These crutches help keep your attention on the page and pressure you to read faster.
Some readers continue to use their favorite crutch for years, especially when they feel distracted.
Phillips, Ann Dye and Peter Elias Sotiriou. Steps to Reading Proficiency.