I think Deadeye has a point that 3 times the mah rating for NICD is probably the fastest you can do with 90%+ safety. Though I did around 2C for my RX NICD pack recently and it barely got warm. I had it on the lowest setting for my peak charger.

The argument isn't what is the absolute fastest you can charge but what is considered by many experts the fastest that can be charged saftly by most flight packs. Peak charging manufacturers tend to be conservative and the battery manufacturers can provide little help in what is the charging limits of their cells.

Anytime you are peak charging cells for the first time, you should monitor them closely and the cells can never exceed 160 degrees Fahrenheit for any reason. My personal belief is that your pack should be no hotter during charging as they get during discharging to ensure max life of cells.