First of all, you do not have a hard drive that can read 200MB/sec. None of us do. You need an SSD to achieve those kinds of speeds. Most hard drives read at about 115Mb/sec and the fastest ones read at about 150MB/sec.
Nor can they write 200MB/sec.
Hard drives read and write speeds change as they store data. Inner tracks are physically smaller, outer tracks are larger. Tracks in the middle are the average speed. We as humans never really know where that data is being written. We just know that it has been stored, and never worry about it.
Drives will slow down if they become fragmented, and you can run Defrag to solve that problem. Fragmentation occurs when data is written, deleted new data is write covering some of the deleted files space, then more files are deleted, and more new files are written... Eventually, your hard drive is fragmented. One pass of Defrag, and its all organized neatly again.