velocity.y = jumpforce(grv(9,8 3) 25) delta, so your jump value is 735 delta pixels.
At 60 (physics, since you are using physics_process there) fps, that is 1000/60 = 16,666...67ms per frame, so a delta value of ~16,67... 735 16,67 = ~12252,45 pixels jumped. Unless I misrecall how delta was reported by the engine, it could also be 1000/60/1000 = 0,01666...67. That would make a significant difference and instead lower the value to 735 0,016 = 11,76 pixels per frame.
Either ways tho, since you are using kinematic_body and move_and_slide(velocity)
you shouldn't multiply the jumpforce by delta when assigning it there and your jumpforce should probably just be equal to your intended jump height. If you intend it to be 12 just assign jumpforce the value 12.0
.
Oh wait, I forgot to account for the spd value... so it's either 12252,45 30 or it's 11,76 30, probably too high either ways.
Anyways, you should probably check this page out:
https://kidscancode.org/godot_recipes/basics/understanding_delta/
and especially:
https://kidscancode.org/godot_recipes/basics/understanding_delta/#using-kinematic-functions