Slow vim startup

I had today a very weird situation.

From my laptop, I ssh to another system, and from there to yet another.

On this final system I noticed that vim starts relatively slow. i.e. this command:

time vim -u /dev/null -c “:q"

returned time in around 3.5 seconds, while on my laptop (which is much less powerful) it is:

=> time vim -u /dev/null -c ":q"
real    0m0.073s
user    0m0.056s
sys     0m0.016s

I tried to debug the situation, and it got weirder. If I did “su – another_user" (on the final system) – it became fast. What's more: if I did su – depesz (my account) back – vim was still fast!

After some debugging it occurred to me: I have automatic X11 forwarding turned on. I checked – and yes, in the shell that vim starts slowly, i had “DISPLAY" variable, set to localhost:10.0!

Quick unset DISPLAY, and suddenly vim starts 0.027s!

Lesson for future – do not use automatic X11 forwarding for long-distance ssh connections, or make sure you run vim with “-X" option.

Later I learned why it tries X11 connection – to get access to X copy/paste buffer (available as * register). Nice feature, but with quite problematic side effects.