Oh my Gawd I can't believe that I missed that!!

Iv'e been working on this customer data take-on procedure this whole weekend (last 36h with 3h sleep)
and there is this one part that was just taking forever.. 

The efficient delete... that now had become ULTRA inefficient...


Below is a Video on how I solved it and what the actual problem was, so if you are interested in solving SQL problems take a look: