In some cases, your graphics card may not support Roblox at the current color quality of your computer. It is t, therefore, recommended that you lower the display’s color quantity.

The FPS in Roblox games may drop if the power saving mode is enabled on your Desktop or Laptop. Similar to smartphones, the power-saving mode limits the performance of your graphics card and CPU ...

This problem can be caused due to different factors like faulty graphics drivers, higher graphics settings on low-powered PC, background programs, etc.