The difference between a shower and a tub is the shower tends to enclosed with water that comes out of a shower head above while a tub has a faucet that pours into a large basin.
The average is 16 inches, naturally there are some lower and some higher, according to requirements and design.
The spout opening on the faucet must be at least 1" above the flood rim of the sink which would be the top of the counter.
48" for a shower and 15" above the tub for a tub.
I generally mount them so the spout is about 3 inches above the sink, but a lot depends on the design of faucet.
When installing a bathtub faucet or tub & shower divertor valve, the bathtub spout must terminate above the flood level rim of the bathtub a minimum of 2"inches.This would place the stub out for the spout approximately 6"inches above the rim of the bathtub.Now just install the faucet or divertor high enough so the trim plate will not contact the spout.12 to 15"inches above the spout should be fine.
According to Building Control Regulations, the DPC should be a minimum of 150mm above external finished ground level.
Replace the shower valve (faucet). I would call a plumber to check it out. Because there may be another reason, it depends on other things besides faucet. Like a crimped water line under slab floor, if you have copper water lines, i have seen the cooper pipe go all the way into a tee and block off the outlet to the faucet. If it has been like i have mentioned above it would have been like this since day one. It would help if people would tell how old there house is and how long has the problem been going on. And yes it could be very much the faucet itself if it just starting do it. IN ADDITION: This also depends if the shower has a mixer or tap (faucet) installed, as the question didnt stipulate this. The rest of the house could have taps and the shower have a mixer. Mixers need a minimum pressure of 400 kpa from the hot water tank (geyser) to work. If the hot water tank is of a lower pressure this would explain for its poor performance.
It is personal preference. Most are probably inside the tub, but for taller people or if the shower head is above the wall of the shower, it may want to be higher.
At least six feet away. The idea is that you cannot be standing in water when you operate it. Most people can't reach 6 feet.
above it in the wall
You can put it at any height. Unless there is some other reason not to, something structural, I always put them above the shower stall for taller people.