OK - here is the whole deal on fast charging NiCads. Forgive the length, there is lots NiCads do.
First, don't take silly risks! Fast charge carefully. Detect all the wrong conditions the battery can get into, and have the charge cut off safely. Know that you may be (carefully) going outside the manufaturer expectation of cell usage.
You can mess with the charging time in a major way.. provided you clearly understand how NiCads behave, the cell capacity condition at the time, and some other tricks.
The small slow chargers, 12, 14 hours or longer, usually only use a resistor to set the current at a value low enough not to badly affect the cell chemistry by continuing to charge beyond the point where the cell is fully charged. This value is usually somewhere between C/10 and C/20.
Several things that are related are ..
1. How NiCad capacity becomes diminished when it is repeatedly partially discharged, then recharged on slow and left overcharging at the "safe" rate.
2. How NiCad capacity is changed when used in series with several cells, and one or more cells become exhausted early because of unequal capacity, and end up being having a reverse current forced through them until the user decides to charge. The "full charge" point of the cells diverges.
3. The effect of nickel crystal growth in the cell, until even it goes short-circuit.
etc.. but we try to stay with charge time...
Provided the "full charge" point can be reliably detected, and you use the temperature detect (diode), and you don't use a current so abusively high that the cell overheats and takes internal damage, you can charge at a rate you choose. The charge efficiency of NiCads is around 2/3, or 66%. This means you add on time to charge the last third.
This may vary a bit depending on the type of cell. Some NiCads have a "sintered" internal feature that greatly increases the cell electrode area, which makes them more suitable for high current fast charge use.
I did once put about 2A charge into a AA cell that was rated to charge at 50mA. It charged in about 30 minutes, and was hot, but not uncomfortably so. I seemed to operate fine, and was abused in this way until I lost count.
When the "full" point is reached during faster charging, the chemistry changes, and the temperature immediately gets going upward rapidly. Even though some fast chargers use this to switch off, it is not a good thing! Only over-charge rates like C/10 avoid destruction!
Aside from keeping tabs on the temperature, to shut down if things go wrong, the way of telling the cell has arrived at the full charge is the delta-V/dt turnover point. Unlike lead-acid batterys, the cell voltage of NiCads changes very little during charge, so voltage is not a reliable indicator of the charge state. It is the rate of change of V while the cell is discharging that is meaningful.
Clever chargers use pulse currents that charge and discharge, with the charge time duty cycle over 90%, and measuring the cell voltage during the discharge pulse. At the full charge point, the cell voltage starts to DECREASE, and the temperature starts to go crazy. I stress that this is what happens per cell. The effect can be masked by the other cells in series. Chargers for portable drills use this, but unless you have access to both terminals of all the cells (like in laptop Li-Ion packs), then uneven charging will happen, and the time to reach the deteriorated capacity shortens. The full-charge voltage also seems to get higher on deteriorated cells.
So - provided you charge smart, and can detect when full, you can charge as fast as is safe. There is lots of information on the internet going into much greater detail about NiCads. For me, NiCads are so expensive, short-lived, and polluting, I prefer other types.