This isn't really a solution to the underlying problem, but
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handles extremely large text files exceptionally well. I've thrown 100MB+ files at it before
(if you think LT's saves are bloated you should see the crap I have to deal with at my day job ) and it always handles things in stride. It isn't free/open-source, but if you're willing to deal with a nagware prompt once in a while it should work fine as an occasional editor.
Just to give you an idea of how it performs, I just loaded a 15.7MB save into it. It took about 2 seconds to load the save and there are no delays in scrolling or editing the file, and searching is pretty much instantaneous.