How did you measure this?

When measuring on one of my servers that exposes both HTTP and HTTPS, there doesn't appear to be any significant difference.

HTTP:

  % wrk http://hostname
  Running 10s test @ http://hostname
    2 threads and 10 connections
    Thread Stats   Avg      Stdev     Max   +/- Stdev
      Latency     5.11ms    5.42ms  62.85ms   96.08%
      Req/Sec     1.10k   224.03     1.31k    87.04%
    17703 requests in 10.01s, 6.89MB read
  Requests/sec:   1768.57
  Transfer/sec:    704.58KB

HTTPS:

  % wrk https://hostname
  Running 10s test @ https://hostname
    2 threads and 10 connections
    Thread Stats   Avg      Stdev     Max   +/- Stdev
      Latency     4.75ms    4.09ms  50.37ms   96.48%
      Req/Sec     1.10k   228.86     1.34k    79.35%
    17014 requests in 10.01s, 8.39MB read
  Requests/sec:   1700.32
  Transfer/sec:    858.38KB