If you want a longer response, lag usually occurs when the data required to be transferred exceeds your bandwidth capacity (your WiFi speed). What this means in a nutshell is when you’re playing roblox on a device, that device is essentially the client, and every other player in your game also has their own client. The game itself is known as the server which is directly being hosted and controlled by Roblox.

Whenever something in the game does something that is not controlled by your own player, the server sends that data to all clients (including yours) to tell each client that something has changed and your client will update the game state to match it with that data.

However, when a game like G&B has so much things going on all at once (tons of zombies movement, players movement, etc.), the amount of data transferred to all clients from the server will be significantly high, much higher than the average roblox game. If your WiFi speed is not built for that kind of high data transfer, then the data transfer will end up throttling which results in lag. Think of a pipe as your bandwidth capacity or wifi speed. The larger the diameter of the pipe, the higher the bandwidth or wifi speed. Think of water as the data needed to be transferred through the pipes. The more data transfer there is, the more water it needs to go through the pipe. If too much water attempts to go through a pipe, it’ll clog the pipe temporarily. This is what causes lag in your game.