I think brake performance degrades so slowly that I don't notice it happening. It was forced into my attention this past week as a problem with rim tape had me changing multiples of tubes on my rear wheel over the past several days. During the process I managed to contaminate the rim with a soap like substance resulting in essentially zero rear brake. Almost stuffed into something before I got on the front.
I soaked then scrubbed the pads in rubbing alcohol but 4300 miles of use had given the wearing surface a glazed appearance. I scuffed them up using a wire wheel and reinstalled. I also wiped the rim with alcohol. First brake application this morning and WHOA!, figuratively and literally. It was extremely powerful, far more so than before I got the slime on the pads, probably like new. Have to be a little careful, especially on slick surfaces, since I can lock it up now.