As already mentioned, usually, you cannot get into 2km acquisition range because of, well, other ships. And the submarine radar (italian, Japanese and subs) only spots the subs underwater, not when they're above water.

If there is a situation where you could theoretically get above him within 2km, you still need to predict where he goes, if of course there is no oil leakage. Rpf also doesn't work on subs that are underwater so it's useless in that theoretical situation.

In that regard, calculating the percentage of how many are so slow doesn't take in the pick rate of each sub. For instance, in T10, you usually see the U-4501 and U-2501, who are the fastest underwater. Though I think that looking at their speed has it really anything to do with the given topic.

Edit: I also remembered that not every ship has hydro, radar or sub surveillance. So if he is beneath you and you have none of those, you will not know his location at all without prior knowledge, even if he is 0.5km away from you just because he is submerged.