As part of my investigation of TLS performance, I decided to benchmark various ciphers and hashing algorihtms on my dev server. My dev machines is a Xeon E3-1220 v2 with 8GB of RAM. For these tests I set the CPU governor performance to insure I wasn’t seeing effects from speedstep throttling the CPU up or down.
The short of it is that I was seeing significantly higher baseline CPU load after enabling H2 on my VPS compared to what I expected. Up from 0.5% to 2-3%. AWS t2.micro instances are burstable configurations designed to operate at a baseline CPU load of 10%. Going from from <1% to ~3% was pretty significant. Not a deal killer, but with no change in traffic that increase in compute load would dramatically decrease the headroom I had to grow before I had to consider a higher tier instance.
I appear to have resolved the production problem by applying the simple principal; encryption strength is proportional to computational complexity, so if there’s a lot of computational load, turning down the encryption strength may improve performance. What I didn’t do was much in the way of actual controlled testing to see if my premise was reliable.