Boiled emulsions were quite common in the early part of the century. Some of them were boiled for hours to achieve final sensitivity and contrast.

The boiling process was most normally done in the absence of ammonia itself, or very low levels. High levels of ammonia in single run ammonia digestion allowed the use of lower temperatures and shorter total making times.

The emulsion in this article also assumed the use of an active gelatin with enough allyl thiourea in it to give a degree of sulfur sensitization which increased speed.

This method was pretty much dead by about 1930, perhaps earlier. Wall makes reference to it in his 20s books but Clark only remarks on them as being older types in his books from the 40s.

Modern emulsions are made with a double run, accelerated growth technique which uses up to 25 steps to achieve very high speed and proper distribution of the iodide.

Slow speed emulsions are always possible with less fuss and bother, but it must be remembered that todays enlarging papers are about ISO 25, and so rank as high speed when compared to those in your reference. They would be about ISO 6 if coated on film. The emulsion in your reference might be the same speed or slower, but would be quite coarse grained if it goes as I suspect.