Before the Battle of Britain began (roughly), Destroyer tactics and performance were somewhat underwhelming. I haven't tinkered with IRAI yet, but relating previous SH games, destroyers start stepping up their game around the end of 1941. During the early war, expect a lack of consistency. I remember getting caught out by a pair of V&Ws near the Straits of Dover (you know...the Deathtrap

) I was on the surface, so they took potshots at me with their guns in the dark. My conning tower got dinged pretty good, but nothing critical. I crash dove, of course, and
one of the two destroyers trundled over to my last position and banged once or twice with ASDIC. I was already long gone before he made it over there.
Enjoy the laziness while you can. Once the Tommies get their act together, they are on you like angry fans at a football riot.