I'll have to try and remember crouching when farming. I usually just trample everything.|
I've tried building a city down at bedrock, called Bedrock, but it just so happens to be Slime Central. When I do extensive builds up on the surface above the frame rate will eventually drop and drop and I'll have to either take a trip (via cart) some hundreds of blocks away so they despawn, or go downstairs and kill them. Kill them ALL.
I usually get about 200 slimeballs each time. You can't actually see how many there are from these pictures, but there's scores of the little bastards in tunnels and rooms and tucked around the main chamber.