How heavy is the pendulum?

Most use a seconds pendulum with a heavy lead bob. The total weight approaches three or four pounds. The heavy bob helps the clock overcome the friction of a large movement. It keeps a steady beat despite vibrations in a busy room.

Why are the dials black?

Black grounds provided high contrast against gilded numerals. Dirt and smoke hid easily against a dark background in rooms lit by open fires. You'll sometimes find white dials on later nineteenth-century pieces.

I read somewhere that all of these clocks were painted over when the taxes changed in the nineteenth century, is that why so many have fading paint now?

Tax changes didn't prompt mass repainting. Flaking isn't a historical anomaly. It is a mechanical failure caused by pine boards expanding and contracting at different rates to the hard paint layers above them. Restorers in the twentieth century often stripped and re-gilded dials. This accounts for the harsh look of some pieces. Finding undisturbed original paint is rare.

My grandfather left a large clock like this and it runs very fast no matter where we place the weight on the rod, could the brass gears be worn out completely?

Worn gears don't speed up a clock. They stop the movement. A clock running noticeably fast usually means the pendulum is too short or the suspension spring is broken. Check where the pendulum hangs. If it swings freely without engaging the escapement, the crutch might be bent. A clockmaker can adjust the crutch and fit a proper suspension spring.