How would you make a roux efficiently?
In a roux, the starches are not normally gelatinized during the production of the roux--in fact, if you make your roux with oil rather than butter, there is no water present (other than latent moisture from the flour) to do so.

Roux thickened sauces must normally be brought to the boil; this is when the actual gelatinization takes place.

. . .

Roux is easy to prepare in larger batches, stove top.  Simple use a larger pan, such as a six quart dutch oven.  

Melt your butter or add your oil, then whisk in the flour.  Cook for a couple of minutes (after the bubbling subsides if using butter), and you are good to go.  Don't be afraid to turn the heat up as long as you are standing by, watching and stirring to prevent burning from hot spots.

Butter can foam up considerably, so use a large pot in comparison to the quantity of roux.

For dark roux, I would start stove top, and finish in the oven to reduce the likelihood of taking the browning too far, and reduce the labor of stirring required.

The roux can be refrigerated.  You can scoop it with a spoon and add it to sauce at the simmer and whisk, and it will thicken almost instantly.

See also:  Is there a technique for making larger batches of roux?