I am having similar issues with documents of similar sizes, on a quite fast machine (see below). I have found that using a large number of comments (I am having 350+ in one document, e.g.) significantly slows it down further, and it seems to me to have to do with Pages auto-updating at which page comments are found. It also seems to me, but I might be wrong here, that showing the right-hand comments-panel makes things worse, while hiding it makes it better. Also turning on and using "track changes" makes it almost impossible to work with documents of several hundred pages.
The auto-updating, smart, behavior of Pages is oftentimes really good, and I find it dreadful to work with other applications not behaving this intelligently at the few times when I am forced to do so, but I think that it would be good if Pages could determine its auto-update rates based on the size of the document and time it takes to perform these background updates, if this is what makes it so slow. Because I would rather keep the flow than always have correct page-numbers in the table of contents and comments bubbles.
iMac(14,2), Intel Core i7, OSX 10.11.3, Pages 5.6.1.
To the team behind Pages, thanx for a (mostly) great application!