The answer is: It depends.

It depends on...the particular private cloud, and the particular public cloud.

And, the data/file type.

I'm not sure there is a general theoretical point here, just something that may be observed in particular cases. I've attached the Enterprise Cloud Leadership Council's 'Workplace as a Service' white paper, which foresees an environment in which many cloud service providers would operate across public and private clouds seamlessly, with security for BYOD enterprises. But that's all promise and case examples, no clockspeeds are reported on Round trips ; )

But good luck - and let us know if you figure out something here at the level of general principle!

Article Workplace as Service White Paper By Enterprise Cloud Leaders...

Network RTT doesn't depend on data/file type, but is a function of the network topology, the physical distance between two locations, and the speed of light. For example, you might find RTT on the order of 500,000ns in the same datacenter, or a RTT of 150ms to send packets from California to the Netherlands and back. See :

http://perspectives.mvdirona.com/2009/10/17/JeffDeanDesignLessonsAndAdviceFromBuildingLargeScaleDistributedSystems.aspx

So I would first be concerned with the physical location of the public and private clouds of interest. Several of Amazon's services, for example, let you buy instances in different geographic regions, and so if you are concerned with minimizing RTT between your private cloud and the Amazon public cloud, you would want to make sure your application was communicating with part of the public cloud in the same geographic area.

Dear “Masterstudent cs”, I’m suspicious (maybe I’m wrong, if so I beg your pardon) you're forgetting the exact difference between Private Cloud and Public Cloud. I suggest you read de Cloud definition as stated by the NIST (http://csrc.nist.gov/publications/nistpubs/800-145/SP800-145.pdf), the one widest accepted, and the different cloud deployment models. Then you will see both of them (Private and Public) can be operated by a third party (the Cloud Provider). Besides in a real Cloud, the platform can be distributed between different DataCentres, so the RTT will depend on what DataCentre is “serving” you at that moment: so in my opinion it’s a very changing parameter for the same cloud and in consequence it’s not useful in ubiquitous and real Cloud environments

Besides, I agree with Murray Stokely, it doesn't depend on data/file type, but is a function of the network topology, the physical distance between two locations, and traffic load. That’s the reason it has no real meaning when you are speaking about real cloud environments (as aforesaid)