Blender—slow to render 100% alpha frame?

Blender fires a light ray into your scene. If there is nothing there to render, it takes a while for Blender to decide to stop continuing searching in thin air and fires another ray into another direction. Before firering a ray, Blender doesn't know yet, there's nothing there.

I believe the distance a ray travels is determent by the camera clipping. If Blender has reached this distance, without finding any objects, it shows transparency or whatever, depending on your render/world settings.

I’m certain someone else can explain this in more technical terms but this is more or less how Blender works.

Also, before rendering, Blender builds your scene for rendering and that takes time too.

That’s why 100 % alpha's, also takes time to render.

If there are a lot of empty frames you might consider to skip them from rendering. I also encountered situations like this and sometimes you need those empty frames to match your clip in the final editing with other clips.

WHY DOES BLENDER TAKE SO LONG TO RENDER FRAMES THAT ARE 100% ALPHA?
What is exactly 100% alpha? Are you busy rendering 100% transparant objects?

I HAVE AN ANIMATION WHERE THINGS GO PAST THE CAMERA, AND I'M NOT RENDERING THE ENVIRONMENT—YET
A few screenshots added to your posting of your render could help to get a nice idea about what you are busy rendering, so we all can "get visually on the same page" and see what you are refering too in your question.

BLENDER'S HAVING TO WORK TO RENDER—ON AVERAGE, MOST OF MY FRAMES ARE ABOUT 6SEC/FRAME, YET WHEN IT'S 100% ALPHA AND NOTHING IS IN FRONT OF THE CAMERA, IT'S STILL UP TO 4SEC/FRAME. 
Without any hardware information, actual render type and settings, scene details from your current project
I cannot make any sense of what you are pointing too.. After all Render-speed is a very subjective thing.

DO I HAVE A SETTING TURNED ON OR SOMETHING THAT'S SLOWING IT DOWN? 
See answer above.. Start by adding screenshots of your render and settings then somebody else -
who is willing to help you, else can imagine your current render-load and speed too.


TIP: "Write to be understood (by others too)"  is a good guideline when posting questions on any forum 

I thought they made themself pretty clear in this post. Not to be rude, but you seem to be searching for a reason to be confused. They are rendering an animation with a transparent background. There are some frames of the animation in which there are no objects in view, so the frame is 100% transparent background. Render times are usually subjective, but they are directly comparing the render time of frames with objects in them to ones that are completely transparent. They are wondering why Cycles can't render a completely transparent frame very quickly, and I think Dolores gave an excellent response. 

MMatt: Exactly that is what you (Matt) assume or think, not what I think.
I'm not deliberately wasting my time and effort to find a reason to be confused as you start to assume here.

I'm more then willing to help anyone on this forum to share my experience and (Blender) information.
Why else would I be willing to join and participate a forum, if it was not for the willingness to help others advance too in there skills Matt, like others are helping me too? got any thoughts about that Matt to share
with me? .

To give you an example of my answering efforts:
I've first analyzed the question and started to find more information around the main argument buried inside
the initial question render frames in 100% alpha.. to find what Recyclops was referring too (since I am not yet familiar with 100% alpha) rendering frame and I've found the following articles, to read and just to get started to understand his question:
1
2
3

I then looked at the rest of this information presented around the main argument to get a very crystal clear picture of Recyclops current rendering situation and challenge.... but came up empty since I the question lacked background-information which could help me to paint a better picture of his situation.  

As you might (or might not) know the thing-called-rendering is (from my own experience with Cycles)
highly dependent on a broad number of very scalable rendering, texturing, scene and object complexity.
Here are a few links to get you up speed on that subject yourself:
- http://boundlessblending.com/blender-fast-rendering/
- https://www.blenderguru.com/articles/4-easy-ways-to-speed-up-cycles
- https://blender.stackexchange.com/questions/44428/why-how-does-tile-size-effect-performance
- https://blender.stackexchange.com/questions/52559/how-to-render-a-scene-faster-in-cycles
- http://wolfig-techblog.blogspot.nl/2015/02/blender-cycles-render-options-review-to.html

So knowing all this last night, I looked back to the initial question, the main arguments and surrounding information presented in it and I found - what I call - a data input error - not enough data to solve the current challenge like - for example :
- scene complexity parameters
- used Render / lighting details,
- used Shaders and their settings
- print-screens of the scene which is being rendered,
- print-screens of the Cycles Node editor,
- No exact data on the rendering hardware that is being used.

Back in the good old days when I was 3rd line ICT tech-support / troubleshooter in a top-100 multinational,
our team-members called this type of questioning helpdesk_input_error, meaning the fact that the end-user
did not provide enough relevant data to us to simply be able to solve their challenge at hand...
This was my main reason why I started to ask Recyclops for more relevant background data.
To simply be able to paint a better picture of her/his situation and therefore provide better answers.

This is also why I wrote the last one-liner underneath my posting:
"Write to be understood (by others too)" 
and never ever assume anything about anything else - but instead start asking clarifying questions.


And now back to you Matt:
What have you done so far to provide any relevant answer to Recyclops's question? 
That is right, correct: Nothing, zero, nada, /null.

The best effort you seem to be able to come up with is finding a way to only criticize others who at least are willing and able to learn, grow and make an attempt to help others on this forum.
One thing has been proven without any doubt for me:
You are not here to help others solving their render problems. Despite you seem to paint / uphold the illusion that you do know what Recyclops is taking about.. you simply fail to provide Recyclops any relevant information (and hide behind Dolores74 who actually does know his render-stuff) which can help Recyclops solve her/his current render challenge nor you make any effort to get his/her situation. So.. I wondering what your true motivations is for your actions here on the forum?


TIP for  the CGCookie forum (moderator) staff:
Suggestion to create an input-template / some sort of forum guidelines which can assist every (new / experienced) CGCookie user on this forum to provide others enough relevant data, so the rest of our community is able to answer their questions (more efficiently and faster).

A very nice place to look for inspiration is the Blender StackExchange - Tour page. They work with a highly efficient way of input-filtering and therefore others can produce lighting fast solutions to incoming user-questions, often within hours on the same day.

Thanks Dolores for the helpful response. It's helpful to know this is normal behavior for Blender!