[Bosje]

That may be related to the early war, I can verify TC not being a guaranteed give-away in 1940, used TC plenty of times waiting for the destroyers to use up their depthcharges some 100 meters behind me.

but

I have also seen destroyers running straight back to my position some time after they had gone back to the convoy. This happened several times when i used x32 compression while NOT silent (2/3 speed, reloading tubes while ascending back up to Pdepth)

Also

I got noticed by a sleeping destroyer parked in scapa flow while i passed close by. that was in stock SH3. I am pretty sure I had silent running on but was making more than 100rpm, he woke up and went after me in the shallows almost as soon as I put on TC (like you, I dont always have the time to sit out the 3 hour sneaking in real time)

So far in GWX, I have used Time Compression without being detected, but always when very silent. (silent running and below 100rpm)

certainly in 1940 this has been perfectly safe for me so far.

My conclusions based on my observations up to this point are:
-Time compression is safe as long as you remain absolutely silent, relative to the enemy's abilities
-If the enemy is theoretically able to detect you, they WILL detect you when you use time compression.

So for later war years i suspect TC becomes unsafe at any time whenever they are actively searching for you, in 1940 you are safe at silent running below 100rpm, whether at 1x or 64x is immaterial.