You have a few options;
1) The first is use a paint roller. You could brush just the grooves and edges near the floors and ceiling then roll the faces using a 3/8 nap roller. This would be much easier.
2) Second, use a paint pad. The same amount of brushing as with a roller. Quality and size of the paint pad will be a big factor.
3) Or use a paint sprayer. An airless sprayer would be the fastest and much easier.
I would use a paint roller. The paint can be worked for a very fine stipple that would be more pleasing than brush marks. Plus it is a bit faster than a paint pad.