I know folks say this with good intentions so I mean no disrespect...but I don't like that phrase. What does it mean? Slow as compared to what?
That phrase is used today because keepers many years ago noticed that voracious baby boa could pound down the chow and get big quick. In some subs it could even lead to litters sooner. Many packed the food in young boas because they wanted a rapid return on investment by selling some trait. In the short term (8years or so) it seemed to work but it was eventually learned that it negatively affected life span and over the course of the reproductive years a boa that was "slow grown" would 'outperform' its fat counterpart. Live longer, higher fecundity...win win.
'Slow grown' as compared to the overfeeding way that didn't work well long term.
Boas should be fed as individuals. The rate changes as they age. The temperatures/seasons offered/daylight times offered/sex/use of reproduction/sub...all these things have influence.