The wonderful site, http://cloudharmony.com/speedtest/run allows you to run tests on latency, small and large file download speed. Depending on the nature of your application, the choice of a provider is crucial (of course price is another important factor).
In the tests, Large File Download category, Amazon S3 is the winner with an average of 15.33 Mb/s. This might be suitable for applications working with large files (pdfs, catalogs, repositories).
Windows Azure Service is the winner of the small file category with around 2.5 Mb/s. The difference between the large and small file is significant. This order of magnitude difference is probably due to the overhead of authentication, establishing a connection etc. Once the pipe is secure and ready, you are probably bound to the limitations of your IPO as well as the cloud service provider's SLA.
Latency might be a concern for high throughput applications - I would argue that self-hosting or keeping the applications close behind the scenes might be a better choice than putting a cloud between your data and application. If it is not avoidable, though, Amazon S3 seems to be offering the best (minimum) latency. This (and the file download times) is impacted by the Geographical designation of the applications and their clients. Clearly as the audience grows, one might want to implement load balancing facilities to route users to the closest cloud servers (S3 offers service in CA and VA for instance)
Great tool Cloud Harmony!!