Roblox physics are expensive to calculate. If you create 1000 parts and weld them all to the character's head in a single frame, the physics engine (Bullet Physics) tries to solve impossible constraints. Result: The server enters a "Physics Stutter," lag spikes to 20,000ms, and shuts down. The Detection and Consequences Roblox is not stupid. They run Byfron (Hyperion) now, which makes executing scripts incredibly difficult. But more importantly, they run Server-Side Anti-Cheat .
Prior to 2014 (and mandatory enforcement in 2016), Roblox had a "Filtering Off" mode. In that dark age, if a hacker changed their walk speed on their client, the server blindly accepted it. If a hacker deleted a baseplate on their screen, the server deleted it for everyone. fe server crasher script roblox scripts
That is no longer the case.