The Capitol, a replica of the one in Washington, was previously the seat of government in Cuba but has mainly sat unused since the 1959 revolution.
Castro then asked how long it had taken to build the Capitol in Havana during its construction in the 1920s.
"A little more than two and a half years," replied Eusebio Leal, Havana's historian and member of the assembly.
"And how long would it take to build today?" Castro asked, referring to the cost overruns and inefficiency endemic to Cuban government construction projects.
"Not less than five," said Vice President Ramiro Valdez, who fought alongside Castro in the revolution and now oversees government public works.
Castro disagreed, saying the current government could not erect the same building in the less than 15 years.