This makes sense! Changed the iteration to: for i in 0..<iterations { let result = flatuseStruct(outputData, start:i) assert(result == 8644311666

This makes sense! Changed the iteration to: for i in 0..<iterations { let result = flatuseStruct(outputData, start:i) assert(result == 8644311666 + Int(i)) total2 = total2 + UInt64(result) } now result is around 43ms Thanks for pointing it out. Have to check if there are so other things involved, but that might be it.