[pit234a]

without knowledge about hardware exactly used in N5200, there is probably no chance to change a processor and gain performance. In other words: you won't see a change in performance with changing the processor on the board.
Why do I claim such things?
Well, top shows me something like this on a fully working N5200:

Mem: 501400K used, 6444K free, 0K shrd, 57220K buff, 415636K cached
Load average: 0.99, 0.80, 0.45 (State: S=sleeping R=running, W=waiting)

and in the following details, almost every process is not giving any load to the processor.
So, as with all devices that may run long-time stable, memory management seems to be more important than CPU performance and that one used on the N5200 is adequate, even more powerful as needed to operate a NAS.

Where do you see a lack of performance?