This issue was happening because there was a think time value specified in the VSM steps.
Not all the steps inside a VSM will honor the think time.
In a HTTP VSM, the following steps honor the think time:
Virtual Service Router, VS Image Response Selection, Virtual HTTPS Live Invocation, Virtual Service Tracking, Live System Failure
The steps that do not honor the think time:
Virtual HTTPS Listener and Virtual HTTPS Responder
The think time specified in each VSM step will be summed up to the response time.
If the VSM executes two steps with 5 seconds think time each, it will send the response after 10 seconds.
If the total think time specified in the VSM is higher than the think time specified in the VSI response, the VSI response think time spec won't be added to the response time.
If the total think time specified in the VSM is lower than the think time specified in the VSI response, the response time will be what is specified in the VSI response only. The VSM step think time won't be added to the response time.
The think scale applies to the think time specified in the VSM steps as well.