How to connect 250mm fan to 150mm coolhood

Filter>fan>coolhood>coolhood>duct running to a whirlybird

My fan is 250mm 10inch
And the coolhood holes at 150mm 6inch
My question is if I use reduces to connect duct from 6inch then reducer to 8inch then to 10inch isnt that going to slow down the fan
Or am I better of getting a 8inch fan ?

Or should I just buy a 6 inch fan just for the coolhood?
 
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