Interesting... I have a gtktextbuffer/textview application which (informally tested) reads an xml file of 3.7MB, and extracts the included text to a text buffer; the text appears in under three seconds, scrollable throughout - so that's included the libxml decoding. The text includes italic and bold information which is also properly displayed, though only a single-sized font is used for this import.
Writing the result as an HTML file (again, via libxml) takes again, about three seconds. The output HTML file is around 600kB. Reading that HTML file takes under a second...
The more changes I make in the text, the longer it takes to save; by the time I have edited the entire text - perhaps a hundred and fifty thousand words - and included titles and headers at various levels, it can take ten seconds, maybe fifteen sometimes, to save - I'm guessing this is to do with internal memory allocation.
The machine is nothing special; an intel core 2 duo at two gig, with 8 gig memory on hand, running Mint 10 linux, with Gnome 2.