How can an FPGA outperform a CPU?
CPU's are sequential processing devices.  They break an algorithm up into a sequence of operations and execute them one at a time.

FPGA's are (or, can be configured as) parallel processing devices.  An entire algorithm might be executed in a single tick of the clock, or, worst case, far fewer clock ticks than it takes a sequential processor.  One of the costs to the increased logic complexity is typically a lower limit at which the device can be clocked.

Bearing the above in mind, FPGA's can outperform CPU's doing certain tasks because they can do the same task in less clock ticks, albeit at a lower overall clock rate.  The gains that can be achieved are highly dependent on the algorithm, but at least an order of magnitude is not atypical for something like an FFT.

Further, because you can build multiple parallel execution units into an FPGA, if you have a large volume of data that you want to pass through the same algorithm, you can distribute the data across the parallel execution units and obtain further orders of magnitude higher throughput than can be achieved with even a multi-core CPU.

The price you pay for the advantages is power consumption and $$$'s.