As the popularity of cloud computing is taking off, so have questions and complaints from IT professionals who are using -- or considering moving to -- the cloud. Industry leaders answer some of the main concerns that they hear from IT staff about the cloud.

Gripe #1: "Wait, this isn't really infinite, is it?"

One of the big draws of cloud computing is the ability to start small and then go big at a moment's notice. But this elasticity should not be mistaken for infinity.

"It seems infinite, but it absolutely is not infinite," says Imad Mouline, CTO of Gomez, the web performance division of Compuware. "In some circumstances, you may not be able to get the instance that you want, the data center or availability zone that you want, or the kind of instance that you want."

You may be exposed to operational risk if your business depends upon being able to provision any level of cloud resources at a moment's notice. "Are you willing to bet your business on it?" asks Mouline. "Or are you taking a chance that on the day that you need to ramp up that you'll have the capacity and that you'll get to it quickly, in hours instead of days?"

If several heavy users of cloud services simultaneously have the same idea to scale up, you may end up in the tech equivalent of a financial panic where everyone tries to withdraw money from a bank that holds inadequate reserves.

Gripe #2: "It's too slow!"

Your computing resources are only as fast as the slowest link of the information supply chain, and if your slowest link is an underpowered cloud computing provider, you may have grounds for a gripe.

"We've won customers that way," says Jonathan Hoppe, president and CTO of Cloud Leverage, a provider of cloud performance solutions.

"Say you've got a website that's going to pull images from cloud storage," Hoppe says. "You want that connection to be very fast, and you also need both to be very fast when delivering to the end users."

Perform some simple performance tests before you start doing business with a cloud provider, and make sure you understand the quality of service and performance guarantees in the service level agreements. "You can tell a lot by looking at the SLAs," Hoppe says. "If they're pretty aggressive, they have a lot of confidence in their infrastructure."

And just because you're using an Amazon- or Google-branded cloud doesn't mean that your website will have the same performance as Amazon or Google. Web performance stems from several factors, including overall web traffic, the location of the data centers to which you've been assigned, and even the behavior of other applications in that same data center. "You can't sandbox everything, and someone else's application may impact the performance of yours," observes Mouline from Gomez. "Performance is all over the place."

You can find Gomez's top-level performance results for major public cloud providers at CloudSleuth.net.