iPhone 4S with latest update now charges very slowly
Plug it in and then power it off.

The charging circuitry runs from the firmware and not the OS - so you will either get a full charge and have a chance to reboot it in 2 or 4 hours to check on the charge status or know where the failure lies.

It's most likely a temporary CPU hog draining charge as fast as it can accumulate and a simple reboot (hold the home button, slide to power off, restart once it shuts down normally) is generally going to clear this up.