You can't compare them directly, and when you use FILE* correctly, it is much faster than ifstream. I think ifstream has a default buffer size which is is bigger than 1, and if you use FILE* and read always 1 byte at a time, then your FILE* is slower. To get FILE* to maximum speed, you need to analyze the media's cluster size and adjust your buffer size to that. There are more things to consider also, like what the cluster size of the harddisk is and/or how the raid was setup.
But a good default would be 16384, if you don't do any further media analysis to optimize the buffer size. Indeed, if you ever have setup raid disks, most OS will suggest a strip size of 16384, because it comes closest to the average file size on a file server, and the strip size should never be smaller than the cluster size on the harddisk.