One of the old BSP's release notes had something about this -- under Unresolved Issues, EDISON-2326 says "SSH slow after GPIO 48 is exported." The Arduino board's digital pin 7 maps to GP48 on the Edison.
I have not seen EDISON-2326 show up under either "Resolved Issues" or "Unresolved Issues" in any of the more recent BSP release notes; it just sort of disappeared. I don't know if they fixed it and the mraa docs are out of date, or if it's still there. I also never saw any explanation of why the state of that one GPIO pin should impact network functionality.
This issue has been reported some time ago. Please check the following thread: Setting GP48 to output or input on breakout will affect wifi
It should be fixed in future releases, but right now there are no updates about it.