I've never done it on purpose but from general considerations and from what I read around, it would seem one is able to slow the growth by feeding less rather than more.
It's, however, both the frequency and the amount that matters. Usually we'd feed an RTC when its tummy goes back to flat and/or they start cruising around searching for food. You might give it an extra day with a flat tummy.
Make sure it absolutely, positively cannot swallow anything inedible in its tank. They do it even when not all that hungry or not hungry at all.
As for the cruelty notion, I'd think it is up to you to decide.
Remember, it's a bell curve: some have good genes others not so much. If you got one from champion parents, it may grow as fast or faster on less food than an average RTC on an average diet, which is about 1.5" a month on average in the first year. If you "lucked out" and got a runt, that'd be the other extreme.
The less food one feeds, the better quality it should be, a complete and properly-digestible nutrition package.
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Growth is also exponentially dependent on temperature. Dropping the temp (still well within a proper range) by say 5F may slow the growth by 25%-50%. At lower temp, it will naturally take longer time for the tummy to go flat and for the hunger to manifest.