Haskell Criterion Benchmark on lazy encode

Question

We're trying to benchmark binary encoding with Criterion. Since the data types are strict, we are perfectly able to benchmark the process of packing a Request.

However, one step further by trying to benchmark the process of encode (Request to ByteString) using runPut we end up getting μs-results. This is probably due to lazy bytestring evaluation.

encode = runPut . buildRqMessage

main = do

  randBytes <- getEntropy 1000000
  let !topicA = stringToTopic "performance-0"
  let !topicB = stringToTopic "performance-1"
  let !clientId = stringToClientId "benchmark-producer"
  let !bytes = [randBytes | x <- [1..1]]
  let !head = Head 0 0 clientId
  let !prod = [ ToTopic topicA [ ToPart 0 bytes ] ]

  defaultMain [
      bgroup "encode" [
             bench "pack" $ whnf (packPrRqMessage head) prod
           , bench "pack+build" $ whnf encode (packPrRqMessage head prod)
        ]
    ]

Is there a way to benchmark the encode process appropriately?