I DID IT.

Succinctly, Zeshy startup previously took ~0.75s on my fairly outdated
machine. That was simply too much, but I'm afraid I struggled to isolate
the issue... until today, that is. Turns out we were reparsing both
"/etc/mailcap" and "~/.mailcap" into suffix aliases on *EVERY* Zeshy
startup, which, admittedly, is not terribly intelligent. Liberally
shifting such behavior to user digest precompilation shaves off an
enviable ~0.50s. Zeshy now starts up effectively instantaneously on
modern machines. Let the giddy elfin dancing commence!