I remember this coming up in the NY Times some time ago. The op-ed writers are researchers in the field, and were skeptical.
http://www.nytimes.com/2016/04/17/opinion/sunday/sorry-you-cant-speed-read.html
So while increasing speed is a good thing, it shouldn’t happen at the expense of comprehension. Anecdotally, I tend to work my way through books at a deliberate pace - It took me a week of vacation during which I did little else to get through Team of Rivals. Even so, I save time by comparison with classmates who finish books in 2-3 days but have to re-read them when an assignment/test arrives.
It all depends on the reason a kid is reading slowly. If they spend a lot of time pondering each passage before moving on, that’s not necessarily a bad thing. If understanding is the issue, then some tricks that improve reading speed can’t hurt, though expecting too much is ill-advised.