I'm currently studying about Linux in general and what it is. I see that Linux is a high-performance operating system that breathes life to old hardware. Ubuntu is a distribution of Linux.
But I'm uncertain whether Ubuntu is an embedded system. Embedded systems execute applications without an overhead kernel, and Android is a prime example in the mobile world. Is Ubuntu an embedded system as well?
Whether it is or is not, how does this fact account for the high performance of the system?
Ubuntu is not an embedded operating system in it's default incarnation regardless of supported chipsets.
As @Rinzwind commented,a mobile build exists specifically for embedded devices.
The performance you are referring to is in relation to Linux as an operating system (more correctly called GNU/Linux as strictly speaking Linux is only the kernel). The performance is gained by keeping the system simple. Obviously if you start throwing windows managers and application frameworks on top then performance is affected as memory requirements increase.
The modular nature of the GNU/Linux operating system allows old hardware to be used where the latest versions of Windows(TM) have excessive requirements just to perform simple tasks. As such, GNU/Linux can provide a more resource efficient system.
Do not confuse embedded systems and real-time operating systems ... the two are often used in conjunction but there is a distinct difference.
Ubuntu is not specifically built for embedded systems, but neither is general linux. Android is linux derived, but it's not exactly linux.
Almost anything can be made performant by stripping out the not neccessary bits. Ubuntu does have an ARM build (which is the typical architecture for embedded devices).
This makes the answer to your question a solid sort of.
I can't think of a better way to explain it. Hopefully this answers what you were after.
Ubuntu is not an embedded system.
That being said, your question leads us to another one, which is: What is the difference between an embedded system and an operating system ?
7 years ago this question would be easy to answer, but nowadays the line which separates these two concepts is disappearing behind a haze... By definition, an embedded system is a small, digital electronic device, based in microcontrollers (or DSPs) that perform specific tasks. On the other side, GNU/Linux (and its distributions) are general purpose operating systems used to do important things like access facebook and your email.
But we are not living in 2005, and based in Moore's law I can say that electronic devices are more powerful and smaller than before. This tendency allowed the electronic companies to install Linux in your TV and car, and in a few years(maybe months) you will have Linux in your fridge, oven and underwear (see internet of things). So, now we have Linux installed in small digital devices doing specific tasks...
Beside my fear of being controlled by machines in a near future, I believe the embedded system definition is getting deprecated. So now we need an international organization like IEEE or IEC to redefine or revoke the embedded system definition. Until that happens you can say that Linux(Ubuntu) can be used in embedded systems applications but usually Linux and its distributions are general purpose operating systems.