Double header this one. My refurbed MTB is no longer pulling me up. I live in a particularly hilly area, bit country and all that

Now when I first fitted MTB generic $12 V brake pads and adjusted it all up it was ok the rear wheel would lock up if need be. I recently rode down a damp hill at around 40-50kph, a road I know quite well; applied the brakes but I struggled to slow down. I ended up with a brake lever to bar moment, whilst pulsing the front brake.
When I had stopped I checked the brakes and found them fine in location from the rim. So thought the problem may be cable stretch, wound in two turn pads were almost rubbing, I still had quite a descent to go. Onto the loose stuff onto a section nicknamed 'pucker up' a loose road section and again the rear would not lock up, not willing to reach hard for the front brake I hung on for grim death and got 4th place on Strava, I weigh 99kg, so I descend quickly everywhere.. Not always by choice!
Now, the LBS say rims need a 'rim stone treatment' WTH is that? I was just going to buy some Kool Stop Salmon pads. Which way would you go....?
MTB has about 3000k's on it and was formally panniered up and toured the east of AU by the former owner. It had little to no pads on it when purchased. New pads have covered about 80k’s.
Also the roadie, 300k's on the Foil, noisy grating brakes on the front, always has. LBS where I purchased from says no problems when serviced 2 weeks ago. Again, the brakes were not very good at biting this morning, not raining but the roads were wet enough to make the tires shine and occasionally spray up.
Could it be the roads around me are greasy/grimy; do I need to up the maintenance/ cleaning?
Any thoughts or suggestions....
Thanks