I always slow down for horses, walkers can move, and old people should be at home knitting. What amazes me though is how a single elderly walker can take up an entire pathway and be completely oblivious to anyone else walking, running, or cycling; they wont move but have a good old moan when you try and politely squeeze past.
I was out riding sunday, moderate speed, and some old dude shouted "OI!! Slow down where my dog is" (his dog was, at that moment, curling out a tyre grenade), to which I replied "Your dog should be on a f###ing lead, and clean its sh1t up you dirty old b##ard".. he stopped and was cursing and was shaking his fist, wanting to fight me. I nearly fell off my bike laughing.