Thanks for the kind words!
Just as a caveat I work more on illustrations and keyframe art in entertainment design than doing a lot of concept art. I mostly work on book and album covers at the moment.
Efficient workflow is something that you have to basically build up yourself from lots of trial and error and learning from watching other artist processes but nothing will trump solid drawing skills. If I were to suggest focus on one major skill it should be drawing/drafting! :)
I have some vids on my YT on what I personally find a quick efficient process for environment designs, which you can check out but it's only one take on it. Mostly what I have seen is that efficient workflows tend to separate out into various distinct "phases" of a design or drawing. You will begin to notice that almost any efficient process will heavily rely on nailing those basic fundamentals first to adequately create a good base to work on. The starter may be an initial line drawing or basic flat value sketches with primitive objects and accurate perspective. You can see some here, though mostly for illustrations I guess
https://www.pinterest.nz/m0nkeybread/art-process/
To create speed most concept artists these days I would say use aids such as basic 3D mockups, photographic base plates, photobashing and such things. There is nothing wrong with this as it is a function of the industry pressures, but for your own skill you have to be careful with relying too much on 3d and photos before your understanding of those fundamentals is developed enough to be able to utilise them well.
For characters silhouette work is often a way used to brainstorm design ideas quickly. Look up the book The Skillful Huntsman for some ideas.
Many people get caught out by this idea of taking the using of aids as shortcuts too early. They might create better work than they are capable of doing from scratch, but if fundamentals aren't up to scratch this becomes blindingly obvious. The aid also can easily become a crutch down the line which results in being deficient in other areas.
So really again I have to stress that I think the best general exercises you can do is to not skimp on fundamentals practice, but also to go through the process of creating an absolute sh*t ton of concept work for yourself and trial out various processes. It won't matter how slow you are if working for yourself and you will get much quicker as you gain "mileage" and learn.
You don't have to give up on becoming a concept artist simply because you feel too slow right now :) Just create a set of specific smart goals about what you'd like to achieve (a vid on that on my yt as well) and work proactively on those. Up your fundamentals skills and understanding as well as doing what it is you'd like to eventually be doing for professional work in your personal work.
Concept art is more about DESIGN, and really about ITERATIVE design, not "art". Being able to come up with a ton of different ideas is probably more important than how quickly you can push out something (though obviously it helps). A good read here
http://howtonotsuckatgamedesign.com/2014...ncept-art/
So you should start to figure out what excites you about the nuts and bolts of designing anything and figure out what inspires you enough to go learn about it and then design something neat from it. Nature is always a great source of inspiration; it really is a masterful designer and many concept artists draw from it for any kind of design.
There are of course many other avenues you can use drawing skills to get work. Publishing (books, tabletop games), editorial illustration, advertising, medical and scientific drawings, toy design, fashion design, themeparks, mattepainting, texture artists. It really goes on and on. Don't forget you can also sell your own work to people direct whatever it might be. The list goes on and on, you just have to find your own way towards something :)
I also second Neos recommendation of Scott Roberston's How to Draw and How to Render. Both will give you all the technical understanding of perspective, form and material rendering you could ever want, but they won't tell you how or what to design :)
If you don't have a sketchbook here, you might want to start one up and post your progress. good luck! Happy to answer questions if I can!