Development below 2 mins
In 2002 I received a developing agent similar to Ascorbic Acid, he named it SEB, it is used for developing microfilms. Developing microfilms is something like 30 secs, his developer is high in Phenidone and SEB plus Carbonate.
So I formulated my own film developer (named K2), using borax as alkaline and ended up around 2 mins, using a Jobo CPE, no problems, fine grain but poor sharpness. The second version (K4) used borax/boric acid like ID-68 and gave times of 4-5 mins, similar grain size and still sharpness too low.
A high concentration of Phenidone gives high initial speed in developing, for normal film the minimum seems to be around 60-90 secs, even 2 g Phenidone could not get below these values with reasonable contrast. I read some eastgerman book that Aminfunctionality gets to the silver quicker than Hydroxyfunctionality, so based on that Phenidone attacks first and the time for this reaction might around 30-60 secs, than the Phenidone gets recovered by HQ and/or Ascorbic Acid. If the later reaction is slower, it would explain the low contrast below 60-90 secs, basically a Phenidone-only developer like XR-1 is pretty low in contrast, HQ etc. is required to build up contrast.
So anything longer than 90 secs sounds reasonable to me, just let me know if sharpness is good, as I gave up on that K2/K4 developers and switched to a staining developer based on Catechol combined with potassium hydroxide diluted 1:1:90 developing times around 20 mins and great sharpness with 100 speed films but grainy with 400 speed films.