Roblox running really slow using a external monitor

You're likely running into CPU throttling issues. If the MacBook is used in clamshell mode (lid closed), much of the heat that is generated by the CPU and other components of the MacBook cannot dissipate properly. MacBook Air models do NOT have an internal cooling fan to cool the CPU but rather, rely on the aluminum case body as well as the spaces between the keys on the keyboard itself to help dissipate the heat.

When the CPU overheats, the system throttles the CPU to a slower speed to regulate its temperature to be within operating limits to prevent damage.

Try to run the MacBook in it's normal open lid mode to prevent overheating and throttling. Another option would be a laptop cooler that you place the MacBook onto. These have built-in fans to help dissipate generated heat.

While heat might be an issue, I would also suspect the hub you have.

It is not one I would choose to use with an Apple Mac.

Belkin, OWC, and Caldigit hubs/docks are designed for macs.

I would also suggest it be a powered dock and not a passive hub to eliminate power being pulled from the MacBook. Pulling power from the MacBook only increases the heat that is generated inside it.

I alternatively suggest you get a USBC to HDMI adapter made by Apple to be sure high resolution is supported.

I have tried off brand cables and adapters and they do not work as well as the Apple OEM adapters. Depending upon your monitor, there are USBC to DisplayPort cables that I have had good luck with, skipping the adapter/dock.

One more thing - all HDMI cables are not alike. So there are some that may not support the resolution and data rates you are tying to use.

What are you using to adapt from hdmi to usbc? Adapter/ hub/dock? Make and model?