"He pasted the backtrace ..." >> Ahem: 'she' [Victoria], actually! ;-)
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Thinking about those enchant-related (spell-checking) error messages mentioned earlier in this thread, I searched/found/read the material provided in these two links,
https://github.com/rfk/pyenchant/issues/11
https://groups.google.com/forum/#!topic/pyenchant-users/QrNRFX3jhEs
that prompted me to examine my dictionary file contents.
I opened my Geany dictionary file (/home/victoria/.config/enchant/en_CA.dic) in gedit -- which has a handy Sort feature -- sorted it, deleted the blank lines (there were a couple: 2 or 3), and saved it.
I then launched Geany from a terminal prompt ($ geany), re-enabled the 'Spell Check' plugin [with each of the four options selected -- as before -- EXCEPT that I now unchecked/de-deselected the 'Print misspelled words and suggestions in the messages window' option], and ran my paste and save steps in the large text file that I've been working on.
Run from the terminal, Geany displayed the same error messages (gtk; gtk tree model; enchant; ...) in the terminal as provided earlier in this thread, but Geany itself appeared to worked fine: quick and immediately responsive (given that I was rapidly pasting in large chunks of output from a PostgreSQL/psql session running in gnome-terminal)!
Ditto, when Geany was launched via its icon (in the Ubuntu Unity panel!
So, this issue may be resolved: practically, if not technically! :-)
Thanks again to all, for your help/suggestions! Victoria :-)
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TENTATIVE SOLUTION:
1. Remove blank lines from your dictionary file. 2. De-deselect the 'Print misspelled words and suggestions in the messages window' option in the 'Spell Check' plugin.