Bad information. There is no better energy saver than the Off button. High starting current is not a "waste" of energy if it is a result of needing to turn something on. Running a motor that is doing no useful work IS a waste of energy, regardless of speed. But it also depends on the process involved.
OK, then why would running at a slower speed help? If you can continuously run at slower variable speeds to match your process work flow rather than at full speed in batches, then you can probably save energy. If the batch process is integral to what you do, then running at slow speed in between batches is a waste. The small amount of extra energy expended in re-starting will not be more than leaving it running at a low speed. In fact, if the low speed operation is only 10 minutes out of 120, then the VFD it will take to run at slow speed for those 10 minutes will end up wasting more than anything you could save when it is running at full speed. A VFD running at full speed will waste about 3% energy.
You would need to define "quick" as it is a relative term. Quick to you might mean 10 seconds, it might mean 1 second or 1 minute to someone else. But as to efficiency, see above.
This is a much bigger "what if" than you may realize. As has been said, starting a 200HP motor every 90 seconds is something you CANNOT do with anything other than a VFD, but even then, the acceleration time would be a big factor because if you must pull more than full load amps to accelerate it fast enough to get useful work from it, then you are still likely to overheat the motor. Either you are incorrect as to that duty cycle or there is something else you are not telling us, such as the fact that the work performed could be done by a 50HP motor but because of this duty cycle, they have put in a 200HP so that it can survive. If THAT is the case, then you can almost assuredly save energy using a VFD because the motor is oversized to begin with.