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if you have HSS on, your flash duration will be a regular flash duration below your sync speed, and above your sync speed it will be the same as the shutter speed. so at 1/250th, you'll have a 1/250th flash duration. if you're at 1/160th, you'll have a 1/8000th flash duration.
Not sure I follow all that but you mention a flash duration of 1/250th. My understanding is flash off a speedlight is much faster than that. To my knowledge, a flash at 100% full power will have a duration of around 1/1000th of a second and as the power is cut [say 1/2 power, 1/4 power, etc.] the duration becomes shorter, into the range of 1/5000th or faster.