Hi George,
asking for the resolution was rather to get a feel for the calculation time. If it’s 640x48, then 15 minutes might be much, if 50 mpx, then it’s very fast! 
The features will be detected only once for each image as long as it’s not altered and the cache not emptied. In the component report it will give you more detailed info about how long each step takes.
Anyway, this isn’t your main point.
I think you’re not the first one to mention that “priors locked” doesn’t work as expected. I might confuse it with something else, but there might be an option somewhere where you can define an image as “undistorted”, which would in my opinion translate in “don’t touch it at all”.