Delta are going to move all their flights to T3 at Heathrow from September, so expect the Clubhouse to get bussier!
Inevitable I guess, and makes sense. DL's J product is pretty good now, and seriously worth considering if you get to use the CH too.
But yes, unless DL open their own lounge at T3 it'll put added pressure on the CH. Most if not all of the extra DL flights depart between about 1030 and 1430 so it would be very busy right up until 1400 daily.
Of course DL might believe their FFs would perceive them in a worse light for not having their own lounge at LHR when their two main competitors, AA & UA, both do, so a new DL lounge isn't out of the question.