Yes. Cast iron radiators typically take longer to heat up than standard radiators that are constructed from alternative materials.
That said, cast iron radiators also tend to retain their heat better than other types of radiator, and will remain warm for longer periods when the central heating has been switched off. They are able to do this on account of the typical greater mass provided by cast iron.
As such, cast iron radiators are often chosen for use as kitchen or lounge radiators.