Found this thread talking about the issue, you may have already read it.
https://www.subsim.com/radioroom/showthread.php?t=84441
The slowing down according the Beery is the lead merchant that the convoy is tied to for speed control is damaged, the rest of the convoy slows also. It appears to be hard coded.
An idea for a mod may be to make a damage proof, unsinkable lead ship, possibly made transparent or invisible in some way or have the torpedoes go through the lead ship without any damage. Then the convoy at least would not slow.
Good hunting,
FUBAR295