Use of the write-blocker is necessary when making a forensic acquisition using a Windows based computer, as the Windows operating system will automatically attempt to mount an newly attached disk drive. (This mount attempt can often result in modification to file meta data, including file access times. It can also alter the filesystem journal.)
The below chart, slightly unfairly, compares the difference in times of using dd to acquire 10GB an 80 GB IDE disk drive via IDE as opposed to acquiring 10GB from the same disk drive, on the same computer, via a USB based write-blocker. It should not be a surprise that the write-blocker took longer, as USB is slower than IDE.
This higlights an important point, which is that the interface to the writeblocker plays an important role in the overall throughput of the acquisition.
Here, the performance penalty of the USB based write-blocker is still evident.