ios - SpriteKit game fps dropping after only a few nodes? -

ios - SpriteKit game fps dropping after only a few nodes? -

i making game has several balls bouncing around screen, , player can add together walls contain areas in balls can bounce. testing have 4 balls, , can separate each ball own bounds 3 walls works fine, no alter in fps. problem occurs when add together more walls, cause unreachable areas covered black sprite node node has no physics. after getting 2 black nodes fps drops around 30. causing this?

here image show mean:

the gray lines walls , black areas unreachable areas

ios xcode sprite-kit skphysicsbody