Only one faucet on my double vanity has low water pressure. The other is fine. What's the usual culprit?

Start by checking the aerator on the low-pressure faucet. It's almost always clogged with mineral buildup or debris. Unscrew it, clean it out, and reinstall. If that doesn't restore pressure, look under the sink for a kinked supply line or a partially closed shut-off valve for that specific faucet. If neither of those is the issue, the faucet's internal cartridge is likely worn and needs replacement.