Today I found a very useful tool. You can monitor the bandwidth of a pipe. For example you copy your raspbian image to the raspberry pi sd card, you can check the write speed.
NAME: pv – monitor the progress of data through a pipe
INSTALL (WITH APT): sudo apt-get install pv
EXAMPLE: dd if=/dev/zero | pv | dd of=/dev/null
OUTPUT: 274MB 0:00:03 [78,6MB/s] [ <=>
WOW FACTOR: 100%