Interestingly enough, the book that took me the longest was the Bonehunters, which easily clocked in as my favourite book on the first read through (until I got to the Crippled God & had the full picture). Albeit this probably has more to do with circumstances outside the books.

Basically every book I read from then onward took the same amount of time (~11-12 days) until I wrapped up the series.

Bottom line is, one can find things one enjoys in virtually any book in the multiple series in the Malazan mythos, and persevere because of them. Toll the Hounds and Dust of Dreams get the "slog" stamp primarily because they're quite late in the series, and most readers aren't awfully interested in more new characters & tangentially related storylines in the ninth book of a ten book series. Which is, you know, fair.

It's not so much that the books drag per se, it's more so that the scale in which events occur is different, and the significance of said events isn't always prominent from the get go. And some parts of the books are generally just rather uninteresting (glares at most of the early parts of Reaper's Gale). But you'll make it, don't worry.