To resolve this issue you should manually remove this counter from being monitored.
1. Right click on your CAS server, Click Detect | Configure Data collection
2. In the right hand window scroll down until you see the MSExchangeWS\Average Response Time counter, highlight it and click the Remove
3. Click OK
This will have to be done for any server that has the CAS role.
Waiting for fix in a future release of Spotlight on Exchange