As batteries age, there current output capabilities decrease. If the phone required close to the max current the battery could produce for it's max burst performance, it's possible that max burst performance would decline as the battery ages. There were a number of articles recently about this happening with apple phones, and apple releasing patches that would slow down phones with old batteries to prevent the battery from being overwhelmed. It wouldn't surprise me if this same phenomenon was present on non apple phones to at least some extent.
Also, as said above, sometimes new software meant for newer, more powerful phones can reduce performance on older phones.