With the same wire gauge (diameter), coil size, and voltage (such as with a mechanical mod), Nichrome will ramp up faster simply because it has a lower resistance thus more power to the coil and a higher heat flux- (Power = Voltage(squared) / Resistance).
With the same wire gauge (diameter), coil size, and Power (regulated mod on Power mode, wattage), in theory ramp up should be the same since the power to the coil is the same, same heat flux, etc. You can test this out by setting up your coil parameters, then vary the wire type in steam engine Wire Wizard, and see heat flux stay the same (Heat flux per Watt of power).
http://www.steam-engine.org/wirewiz.html
In practice I find this appears to be true, I use the same power settings for the same gauge wires and coil size, whether kanthal or nichrome, or SS.
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The biggest factor in ramp up time is actually wire gauge, larger diameter wires (lower gauge #) will heat up slower for the same Power level, since the heat flux (mW/mm²) for bigger wire will be lower for a given power due to the larger mass that has to heat up.