Unlike the Musketeers he found it unsufferable as a kid, could only finish it by constantly skipping ahead.

His fascination with the novel came after he was commissionned to abridge it and realised a hundred pages in that it just didn't work. He investigated why it didn't work, and the result was that essay in "Supermen".

I don't think (I could be mistaken) it's Eco himself but rather another commentator, quoting Eco's essay who said that getting frustrated at the lengths and redundancies and giving yourself the permission/pleasure to skip parts is part of the experience of reading Monte Cristo.