I was suspicious, I just could not put my finger on it :)

   for i in 0..<iterations {
     let result = flatuseStruct(outputData, start:i)
     assert(result == 8644311666 + Int(i))
     total2 = total2 + UInt64(result)
   }

this results in around 42ms compared to C 25ms. I guess I should update my blog post :)

Thanks for your help.