What does that mean?
If the B”E believes that not learning up to your full potential is bittul Torah, then my prep for laining is bittul Torah. You can make the argument that it might be necessary, but you could probably also make the argument that, perhaps, it should be done by someone for whom it isn’t bittul Torah.
Again, I don’t believe any of that to be true, but it is the logical extension of the B”E’s opinion.
The Wolf