Latency in this case would be the time it takes for your device to ask the raspberry pi what IP address is www.google.com and the pi will reply to the device with the IP address.
I think you might be misunderstanding how the pi hole blocks adverts.
It doesn't filter http traffic by looking at the contents but instead has a block list of domains that server adverts.
When a device on your network requests the IP address for a blocked domain PiHole just replies with a fake IP (or nothing, I'm not sure)