Apple did not provide any hints as to how fast, when it comes to clock speed, any of its system on chips are, leaving the tech community to guess the numbers.
Anand Lal Shimpi & Brian Klug from Anandtech took a number of performance results from the A5 that have surfaced around the web and compared it to the database of test results compiled by Anandtech.
Ditto for Rightware Browsermark which shows the remarkable jump in performance from iOS 4.3 to iOS 5. The iPhone 4S comes ahead of the Galaxy Tab 8.9, which is a tablet and rakes in more than twice the BrowserMarks of an iPhone 4 running iOS 4.3.
It is when compared to the iPad 2 that Anandtech came to the conclusion that the iPhone 4S is indeed running at a slightly slower speed than the iPad 2 (800MHz vs 1GHz).
In any case, some assumptions are made including that performance varies linearly with clock speed and the same scenario holds when comparing the GPU performance of the iPad 2 and the iPhone 4S.