over 1k to blink a LED ?!
Old but still appropriate analysis here: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1283329855
(Arduino has featureitis. An empty sketch has grown about 50% since that message.)
As for A2D conversion... A successive approximation A2D converter theoretically gives you 1 bit of resolution per clock cycle. That is, after one clock, you know whether the input is over or under Vref/2, which is exactly the same as 1-bit resolution A2D conversions. However, I would not expect an A2D "peripheral" buried inside a microcontroller to allow the SA shift register intermediate values to be accessed by the CPU; probably that is transferred into CPU-readable registers at the end of the conversion.
The variable timing of A2D conversion probably comes from this part of the datasheet: "By default, the successive approximation circuitry requires an input clock frequency between 50 kHz and 200 kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the input clock frequency to the ADC can be higher than 200 kHz to get a higher sample rate." So you can set the clock fast enough to get the 13uS conversion, but your accuracy will suffer.
dedicated clock domain.
A dedicated clock domain
is not the same as a completely separate clock. If you look at the section describing the system clock (sec 8 in my manual), you can see that the A2D clock is derived from the same system clock as the CPU clock (it's just derived separately, so you can turn of CPU clock while leaving the A2D clock running.)
before insulting people
There's been a rash of recent postings by people with "experience" showing up and using their first few postings to complain about things they don't like. "The debugging sucks." "The IDE needs a complete rewrite." "Where's my C++11?" "The code is too big." "The functions are too slow." "The board and shield design is lousy." "Arduino should use a more powerful CPU." "The Due is late." "The documentation doesn't include the full C/C++ standard includes and libraries." "I need an Arduino core for <processor that has never been supported.>" "Raspberry Pi looks a lot better." (no, not all of these are you...)
It gets tiring. It's not that none of these complaints have any merit. It just that most of them have been discussed over and over again, and are relatively orthogonal to the "purpose" of Arduino anyway. It would be nice if new users, especially the ones with existing experience, took a while to look around and pick up the style of discussion, FAQs, and ... "the spirit of Arduino" before launching into complaints... Detailed questions on the operation of the A2D converter, beyond the AnalogRead() function, are more appropriate to AVRFreaks (where they're even less ... tolerant.)