Why might 1st byte be much slower on second run?

See this report: http://www.webpagetest.org/result/170602_7E_X3X/

The first run is great. Love it. The second run is a disaster. This result is reliable over many months of samples. In fact, first-run time is trending down over the last few weeks while second-run time is trending up (by a LOT).

I don’t understand why that might be. Any idea what would make the cached load take so much longer to get off the server?

Do you have page caching installed?