If you wish to cut the development time in half, you need to raise the temp about 10C (or 18F). (This MIGHT be a bit too much, however, depending upon your developer formula which, in the case of Ilfotec, I am not privy to), OR, instead, you could add a bit of sodium carbonate to the developer and use your standard 20C (68F) temp. How much carbonate? You will have to experiment by exposing a few cm of film (why waste a whole roll?), but I would start at about 1 mL of sodium carbonate added to 500 mL of working solution developer.
You want to increase the energy of that developer, marciofs, and that is accomplished with either a temp rise or a more alkaline developer solution. But, you COULD, also, increase the energy of that developer by increasing its concentration. If you are currently using a 32X dilution, as you are now, try, instead, with only a 10X diultion (i.e., 1 + 9). - David Lyga