It is not a technique that I dreamed up myself. I got very close to 1 stop increase in film speed. And I got it with regularity. However, I got even more with low level light...asa 64 to 200 with 100TMax and an icrease in shadow contrast. Try a shadow based exposure and try again with two 1dentical placements on Zone 2 or three and see if it helps. I first picked up on this technique from an article written by a Mr. Pericano...I hope I spelled his name correctly. If my memory serves me correctly, One article was about monobath developers, another was about kallitypes and a third was about desenisitizers and there may have been a fourth. I believe that the monobath article mentioned it but I don't know which of the other 2-3 mentioned it. All of the articles appeared in the finest..my opinion..photography magazine extant. Photo Techniques. What else would one call a photo magazine that publishes those neat articles by Patrick Gainer? I should have been a bit more moderate in my statement and have said that it can add up to 1 stop of film speed.
I believe that if you use Google and type in "film latensification" or "sodium perborate" you can find other information useful to you about it.