Well, my DR is definitely having trouble rendering complex pdf files.
Your intuition is right it's not so much about document size, it's about complexity. Combining layers of text, vector and bitmap elements are lethal.
The buffering algorithm needs to be unproved: e.g. with a 25 MB pdf my DR needs 30s+ just to render some pages (remember that's with the CPU at full speed...), and more often than not it will just hang, requiring a reset.
I suppose the good news is that it can be fixed. Let's hope it's done sooner rather than later.