Hello and Welcome
Can I ask if this TC slowdown occurs when ships are sunk in shallow water?
It has been shown that if a ship is sunk in shallow water then the SH3 engine still tracks it's existence.
FYI - cruising around at high TC, whilst quicker, means that you miss a lot of action. I personally only use a TC upto 512 and only when going to America. At all other times nothing above 128.