Bug: Level Up causes stuttering

[Alyosha-ctrl]

When leveling up in tower_survival.gd. There is a noticeble stutter after leveling up if a lot of cube's are visible at once. Additionally there is a thing where at faster speeds this happens as well. Fix this through any means you have wether it be performane increases or setting up a level up screen to disguise the stutter.

[Alyosha-ctrl]

Somewhat lowered performance issues by lowering the physics tick rate, solver iterations, and making entities invisible if they aren't on screen