Recently downloaded several 2-4GB file sets full of a few thousand small files each, and noticed two things:
1) The UnRAR process took a LONG time. I mean it took hours to unrar these files, where using an external tool to do the unrar only took a minute or so.
2) While the UnRAR was in progress the system was pretty much non-responsive. CPU was running between 50-80% of total, and any UI interaction had huge delays. If I paused the unrar (after several minutes for it to respond to clicking the pause button), everything went back to normal. Unpause and everything went back to unresponsive (and un-rar started from 0% again).
I've never noticed this on similar sized RARs that contained only a few large files.