A darlingtons switching speed mainly depends on the external circuitry and the technology, it's not generally very slow, or much slower than a regular bipolar transistor. The worst switch parameter with LT1074 is the high saturation voltage, which is implied by the (actually three transistor) darlington configuration used with the part. The basic part has been brought up 25 years ago, I'm not sure if LT1074 or a similar predecessor, I evaluated it and disliked it from the start.