AFAIK, the sensors may become an issue only if potential targets get in range of the mines. Steve had a lot of slowing issues during his multi-nation games.
"Potential targets" obviously refer to enemy ships, in which case the slowness shall occurs only once, but I'm pretty sure that neutral and friendly NPCs will also trigger a detection event from the mines, without detonation...
However, having a lot of active munitions loitering around may lead to slowing issues by itself, regardless of the sensor part of the problem.
