Do electric kettle's take longer to boil in the USA due to lower mains voltage?
It's not just you, and you are in fact correct.

What it boils down to is the available power on the local circuit breaker.

In the US, a house is normally broken up into various segments, each getting it's power through a 15A circuit breaker.

As such, the maximum available power for the circuit is 120V * 15A, or 1800 watts.
Furthermore, to prevent the breaker from tripping if a lamp or other device is also plugged into a circuit, the maximum power I have seen a single device draw is generally limited to ~1500W.

In a country with 240V mains, you only need to draw ~6.25A to get an equivalent 1500W.
A quick search for 240V electric kettles leads me to some devices that claim *3100 watt heaters. 

I don't know what the standard/common breaker rating is in a 240V country, but if it's also 15A, that would mean that you can draw approximately twice the amount of power from any one outlet then you can in a 120V country.

That likely explains the difference you perceive.



Edit:

From @RJR in the comments:  


  Common breakers in Europe (NL at least) are 16A. 10A (older homes,
  lights) and 20A (power appliances like hotplates/hobs) are used also
  but not generally for normal power outlets.


As such, the maximum available power for a electric kettle is indeed twice the maximum available power in most 120V houses.