is 'dd' command taking too long?

In the future, you should use pv to get a running progress bar.

sudo apt-get install pv

With pv installed, let's assume you want to clone a 20GB drive, /dev/foo, to another drive (20GB or larger!), /dev/baz:

sudo dd if=/dev/foo bs=4M | pv -s 20G | sudo dd of=/dev/baz bs=4M

Important bits to notice: the bs=4M argument sets the blocksize for dd operations to 4MB, which drastically improves the speed of the whole thing. And the -s 20G argument tells pv how big this operation is expected to be, so it can give you an ETA as well as a current speed.

I love pv so hard it should probably be illegal.

Note that while doing it this way is intuitive and nice and neat from left to right ordering, piping to and from STDOUT can incur a performance penalty if you're talking about really fast data streams. The following syntax is faster, if you're looking at moving several hundred MB/sec:

pv -s 20G < /dev/foo > /dev/baz

The -s 20G is optional, if you actually know how big (or about how big) the stream will be, it allows pv to give you a time estimate for completion. Without that, pv will try to figure out how large the dataset is if possible (for instance, it knows how big a file is) but if it can't (eg with a block device, not a file), it just tells you the rate of transfer without guessing how long things will take.

You can see how far it has got by sending it a SIGUSR1 signal in order to see how much data it has copied and the transfer rate:

kill -SIGUSR1 $(pidof dd)

For copying activity you are limited by I/O speed of the device, so the CPU should not be fully loaded, so don't worry about that.

I've used pv as well as (ps and kill) in the past as suggested in the other answers, but more recently I've just been using dc3dd instead which produces the same results while providing a progress report throughout the process.

You can check to see if it's already installed with: which dc3dd

If not you can install it with sudo apt-get install dc3dd

The command switches are similar to dd (for cloning, although wiping is a bit more straightforward).

In your case I would use the command dc3dd if=/dev/sda of=/dev/sdb

Edit:

Recent versions of dd from the coreutils package version 8.24+ included in Ubuntu 16.04 and later include a status parameter. You can accomplish the same result with dd by adding the status=progress switch to your dd command line.

Example: dd if=/dev/zero of=/dev/null count=1000 status=progress