Will my iPhone charge slower if I connect it to my laptop?
Most computer can’t charge an iPhone as rapidly as the dedicated wall chargers sold by Apple. The exception to this generalization is some macs do charge iOS devices rapidly. The iPhone 4 and most other iOS devices are designed to charge at higher currents than the baseline USB can power. 



Many newer Apple products will provide up to 1100mA of charging current at 5V even though the USB specification limits current to 500mA at 5V.



This will certainly charge an iPhone 4 faster than most computers or "by the spec" USB chargers. The The iPad 10W charger is much faster than the iPhone charger so I don't know who would win in a race - a mac with extra operating current, the US iPhone adapter, the UK iPhone adapter or perhaps the Swedish iPhone adapter

The Mac/USB bus details are documented at Apple Computers and Displays: Powering peripherals through USB as well as in the iPad: Charging the battery article. The nice picture is from the "Powering through USB article".