I have crappy water pressure in my shower. However, the sinks in my bathroom and kitchen have no such problem. The sinks have the fury of a geyser, compared to the anemic mist the shower puts out. My high-tech measuring methodology is this: in any of my sinks, I can fill up a pint glass in about 1.7 seconds, with either hot or cold water. However, if I use either the bathtub faucet or the shower (with the shower head removed, so it's just a bare pipe) this takes about 8.2 seconds.

So the water is there, it just ain't making it to the shower. The sink looks like this, and the valve in the shower looks like this:

They appear to both be fed by the same 1/2" flex-tubing. The two screw-valves on either side of the shower valve assembly are turned all the way up.

I had a plumber come out and he said that he could replace the shower valve assembly (which would require turning off the water to the whole building, knocking out the wall, and re-tiling), but that it likely wouldn't make any difference, so I should just live with it. He seems to suspect that all such valves are standardized at a "crappy water pressure" setting, and replacing it would leave me exactly where I am now.

I can't see any markings on the thing in the wall that would let me research what its flow is actually rated to be, so that I could compare it with potential replacements.

Any thoughts?

Update: I took out the flow regulator that was hiding behind the big screw thingy that is 2nd from the right in the right picture above (it's the smaller part pictured to the right, here), and now I get 1.7 second flow from both the tub faucet and the showerhead pipe! Unfortunately my shower head still retards that down to around 4 seconds, so it's time to find a better one of those...

Update 2: I spoke too soon. It seems that taking out the flow regulator increased the flow of cold water but not of hot water, so after taking that out the shower was slightly-warm at best, never hot. So apparently I get to choose between good water pressure and hot water.

The hot water valve on the left doesn't open as far as the cold water valve on the right does (one turn versus two), so maybe that's the problem. But replacing that would definitely require opening up the wall and de-soldering the pipes.