Does Roblox Allow Using FPS Unlockers?

Roblox does not officially support third-party tools including FPS unlockers. 

Some FPS unlockers may lead to wrong behavior and glitches. This is so because may might affect Roblox's frame rate management directly.

Technically not forbidden: Roblox does not have an official policy declaring FPS unlocking forbidden. Their terms of use, however, forbid using illegal programs that might give unfair advantage in games.

Alternatives you can consider:

Roblox itself has a built-in setting on the graphic options menu known as "Maximum Frame Rate". It lets you modify the framerate without using third-party tools.

Best advise: 

If your computer slows down when attempting to keep a high framerate, think about changing your graphics card or CPU. This will be the best way to attain smooth gameplay. Using FPS unlockers is a bit of a gray area generally. Although they are not specifically forbidden, Roblox does not officially endorse them and this might cause issues. To get better game performance, either update your computer or gadget or use the settings inside the Roblox gaming app.