Render Issue, is 60% normal? and why does it cause lag so much?

my game which is not so big on a scene, which it contain like 128x128p tower object that can be purchased, 64x64p wall that can be build, 1 huge 1920x 1024p sprite that did nothing as background, and enemy that is 96x96 object that spawned when the game started, and its number is increased for each 2 wave and start with 5 on the first wave.

but as the game progress the game become lagger and lagger, at wave 1 its 60 fps and at wave 11 its 25, i check on my profiler and notice that the Render usage is so high, around 60% when normaly at the start of the game it was around 25%. something to note that at wave 11 i got like 11 Tower and 25 wall.

so i decide add command to dlete those obejct at wave 11 and check the profiler and its the almost same. Is there something i dont understand about render on gdevelop?
any help would be appreciated!