Low water pressure used to be something I only experienced visiting third-world countries. And the rare occasion when an extended power outage prevented water pumps from refilling water towers (e.g., New York City on August 14, 2003).
First world water infrastructure is supposed to deliver potable water at a pressure of 60psi. Water conservation fanatics came along and thought our 1/2″ pipes and 3/8″ fixtures were delivering too much water to faucets and showers, so they pushed flow restrictors.
When I stay at hotels the showers are often so weak that I wish I had brought a five gallon bucket to shower from (like I did in some areas of Mexico). Instead I now pack a small wrench to remove the shower head so I can pry out the flow restrictor.
Often the restrictor will be a small plastic disk like this, in which case all you have to do is pull out that black O-ring and regular flow will be restored.
Higher-end restrictors might have more pieces, but they can all be pried out to restore flow.
