Can FPGA out perform a multi-core PC?
A colleague of mine benchmarked this and came to the conclusion that FPGAs would outperform a PC once you had more than about 100 independent, integer tasks that would fit in the FPGA. For floating point tasks GPGPU beat FPGA throughout. For narrow multithreading or SIMD operation then CPUs are extremely optimised and run at a higher clock speed than FPGAs typically achieve.

The other caveats: tasks must be independent. If there are data dependencies between tasks then that limits the critical path of computation. FPGAs are good for boolean evaluation and integer maths, as well as hardware low-latency interfaces, but not for memory-dependent workloads or floating point.

If you have to keep the workload in DRAM then that will be the bottleneck rather than the processor.