Deploy TCP Fast Open at exits (and maybe inter-node?)
Most of our network runs on Linux, and TCP Fast Open (https://en.wikipedia.org/wiki/TCP_Fast_Open, https://tools.ietf.org/html/rfc7413) has been supported by Linux since 3.6, and enabled by default since 3.13. You have to use special socket APIs on the client side to use it, though, so we need to patch Tor to make use of it.
If we turned this on at Tor exits, I would guess it would make most of the exit connections 1xRTT, since cookies would be shared by all clients using that exit, and for popular destination servers, odds will be high that a given exit has connected to server recently.
I'm not sure the inter-node case will help as much, but maybe.. However, if we do use it, we'll need to be extra careful not to use it for Tor clients (or bridges), to avoid linkability.