Asus tuf GTX 1650
that game ran well for my brother for months,If you can limit it to a single game, then it is likely the game.
Could be as simple as that Roblox specific game being poorly written. Do other Roblox gameplay types suffer the same problem?
First step would be to update all the system drivers. Starting with the motherboard chispet, audio, and network. GPU drivers.
GPU drivers is where you might want to experiment with using an old driver.
If you have Geforce Experience installed, make sure nothing weird has happened to the profile for Roblox, like dynamic super resolution. Check the Nvidia control panel for any out of place settings as well.
When you say only Roblox, do you mean everything in Roblox, or just that specific game mode in Roblox?
1- gpu drivers are up to date whichwith the B450, you have a Ryzen CPU, which *really* wants 3200/3600 RAM.
have you done driver updates? Is is possible Roblox has VSync on (and set to 30fps)?