I don't think you really want to slow the belt down. You just need them scaled correctly. Let's say that the maximum cornering G of the vehicle you are simulating is 2.5G. The you want the G-Belt cornering effect to be at it's maximum when you hit 2.5G. In that way, it will only have moved 50% of its travel at 1.25G, etc..
The easiest way to get this with no effort at all is to simply use cloud tuning. With cloud tuning, we automatically fill in the maximum G your simulated vehicle will achieve for a given track. You just click, drive and it will behave as described above. You can learn more about cloud tuning here:
How Do I Use Cloud Tuning? | SimXperience® Full Motion Racing Simulator Technologies
What Does Cloud Tuning Do? | SimXperience® Full Motion Racing Simulator Technologies
If you are not using a cloud-designated profile (one with the cloud icon in the top-right), then you can manually set the peak cornering G of the vehicle by expanding the Cornering effect and manipulating the maximum G-Force slider. See image showing the peak cornering force at 2G:

It will be best to start with a default profile. If you've been messing with other settings in your profile already, you can easily reset it to defaults to make sure you didn't make another change that is causing your issue. Here is how:
How To Reset Profile To Defaults (simxperience.com)