Are you running the game on a monitor that's above 60hrz?

The game's physics is tied to the refresh rate of your monitor. So for example if you're using a 144hrz monitor the game will run at 144fps. Now normally this would be a good thing right? More frames per second means the game runs smoother right? Well in this case no, The game at normal speed is supposed to run at 60hrz or fps at all times. Anything below 60fps will cause slow down and anything above will cause the game to play too fast.

Now that we know what's the cause of the issue the fix is pretty simple.

To fix the framerate issue do the following:

1. Open NVIDIA control panel.
   
2. Under the 3D settings list go to "Manage 3D settings"
   
3. Select the program settings tab
   
4. Click the drop down menu under "Select a program to customize" and look for soniccd.exe . If it's not there you'll have to manually add the program to the list. This can be done by clicking the add button and locating the .exe file where you keep your game downloads. Eg: C:\Steam\steamapps\common\Sonic CD .
   
5. Once you added the .exe, scroll down and select the "max frame rate" option.
   
6. Select the slider option and adjust it to "60"
   
7. Select ok and don't forget to select apply

Once you're done with that the game should return back to normal speed and be playable again.

Have fun!