This is from Gate's web site.

How efficient is the system when compared to a chain?

Repeated testing has found new belt drives equally efficient to new chain drives, including a third party test evaluating the Carbon Drive system on a bicycle. Currently, independent testing is being conducted with latest production Carbon Drive components under additional conditions to further confirm the results.

Maybe I will ask them for copy of the test results to back up this claim.

I have not heard anybody claim its slowing them down.