I faced the same problem after having had a window manager compositor crash my Xorg session (or at least my window manager), kicking me out of all my applications in the process. In my new Xorg session I opened a new terminal and reconnected to my existing tmux (2.8) session, but noticed that I now had that delay in starting Vim, exactly one full second in "xsmp init", only when using Vim through tmux. It's such a perfect second that I suspect it's a timeout, perhaps due to tmux trying to connect to my previous Xorg instance.
What worked for me: I exited my tmux session entirely, confirming that no tmux process was still running, then I launched a fresh tmux session. Delay gone!
So I guess tmux has problems with an Xorg session disappearing from under it.