As a sector goes bad by a life expectancy count (they all will over time) the drive opens a back up sector to replace it, and writes the data to it and the back up sector becomes the new one that is used instead of the bad one. The bad one is done, it's not used anymore. As this happens over time (year or two) your drive uses up good sectors and marks them bad.

Operational speeds dramatically decrease once you have millage on the drive. Most tests are done with clean slate SSDs and that leads to peak performance numbers, but not real world long term numbers.