Steve's point about mental exhaustion is well-taken.
I tend to teach students that rhythm is not so much a matter of (measurable) time, but a spectrum between indecision and indecision - meaning, at one end of the spectrum, to play before one has made up one's mind, or, at the other end of the spectrum, second-guessing after one has made up one's mind. Probably needless to point out, the former's too fast, the latter too slow - that's where the time aspect comes in.
Even so, consistent high-runners tend to be players who have been on top of the mountain, bashed it all out with God, so to speak, come back down that mountain, and since then make comparatively few conscious decisions anymore ("seen that, done that").
Mental exhaustion can be a problem in particular for the more attentive and sensitive, not to mention intellectual among us - having said that, it can genuinely add to the fun of playing at all to perceive everything (situations in this context) as new and unique, as well as assess their familiarity. I personally don't think I'd still be playing after 25 years if I saw it all as an endless repetition.
From a psychological perspective, I would like to add that doing things "one's way" (= principled) adds pressure. Why not enjoy the freedom of doing it differently, give in to one's whims, living the moment so to speak, that is, feel free to mull over a problem when so inclined, and speed up when feeling like a spring chicken (or, for that matter, bored with all the detail)?
Many if not most great players I've witnessed have a tendency to sift through the first rack or two deliberately, following which their pace tends to pick up a bit (= after their fashion), and many repeat this each time they come back to the table (i.e. they will not rush into a new inning, but acclimatize anew each and every time).
But: you'll need to do things your way - I think that is the secret to Straight Pool.
Greetings from Switzerland, David.
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„J'ai gâché vingt ans de mes plus belles années au billard. Si c'était à refaire, je recommencerais.“ – Roger Conti