General rule of thumb. 1 HTTPS request = 2 HTTP requests from a performance perspective.
Since we measure everything in requests/second when we size, that's roughly how I factor it in.
If you had a proxy averaging 100 Req/Sec of HTTP and 50 Req/Sec of HTTPS when you aren't decrypting (150 Req/Sec total)
Then that would be about 200 Req/Sec when you are decrypting.
Thanks very much for the feedback. It should help us make a decision