RFC822 items can be big, and correspondingly expensive to manipulate. tcllib's mime package parses any presented item completely. What if one only wants, say, the "Subject:" of the item, while needing performance? In principle, there ought to be a "lazy" evaluation that intelligently scans the few header lines necessary to determine this value.
A comp.lang.tcl thread [anyone have a reference here?] discusses this. One highlight is that Don Libes's "mail notification tools (i.e., biff)" page on tkbiff points to code which directly models such a requirement.