[DigitalAura]

couple of things:

When you say your computer slows down... do you mean your TC bumps back to 1X? Or do you mean you sense a hesitation as your CPU registers the extra processor load of the extra data of having a convoy nearby?
This would be difficult to gauge, as computers are all different and my computer may not slow down at all.

Also, do you notice differences based on individual crew members manning the sonar station? I do. In fact when I call BattleStations I seem to get much better results... perhaps because the one crew member has a special ability concerning PASSIVE SONAR. I don't know, but going Battlestations for me produces better results.

I have experienced this too even after spotting the ships and, as you say, I can hear them plainly myself. I think the important thing to note here is that the sonarman WILL NOT report a contact if he can't distinguish between Warship and Merchant. I've noticed that the instrument that reports bearing and TYPE (in the top left hand corner) reads 'unknown' as type in these circumstances. Perhaps the problem isn't that he doesn't hear it, but doesn't report it until he can ID it.