Here's TeX's line-breaking approach (as I understand it) in a nutshell:
\pretoleranceis positive, try to break a paragraph into lines without inserting discretionary hyphens and without exceeding a badness of
If method 1 fails, allow hyphenation and try not to exceed a badness of
If method 2 fails and
\emergencystretchis positive, try again with the amount of "tolerable" white space per text line increased by
On p. 96 of the TeXbook, Knuth reports experiments showing that "the first pass [without hyphenation] succeeds more than 90% of the time" for "fairly wide" lines, but fails quickly for "very narrow" ones. He also states that the first pass is done "[i]n order to save time". My interpretation of this is as follows:
For cases where line-breaking without hyphenation fails, one would in fact save time by omitting the first pass (i.e., setting
It is also possible (though maybe not very likely for languages with a small average word length) that the first pass will succeed, but that allowing hyphenation would have resulted in a solution with smaller badness.
As Knuth nevertheless chose a default value of
\pretolerance, he must have regarded the net time savings from "trying without hyphenation" as worthwile, given the average processing power of the time when he adopted these settings.
I don't know if the default settings for TeX's line-breaking algorithm changed over time. But isn't it possible that formerly substantial net time savings are irrelevant today?
So: Is it still worthwhile to let TeX try line-breaking without hyphenation? Or is it preferable by now to adopt
\pretolerance=-1 as default setting?
It does not have any serious impact on performance on modern machines and I can vouch on old machines as well. Depending on your settings more than 50% of text would normally pass through the first pass. Here is a figure of two tests (the red numbers denote badness):
The tests were carried out using code posted by Wilson on Git. Personally I would recommend let the \pretolerance stay at 100 it will probably be faster (as you do not force the other passes in the majority of cases).