Votel noted that the operation to retake the city of Mosul in Iraq from the Islamic State took nine months.
By comparison, the terror group has had years to prepare for the Raqqa offensive, Votel said.
The CENTCOM commander also noted that the US-backed forces fighting in Syria are not supported by the government there, while the US partners in Iraq are.
"They don’t have all the trapping of a big army," he added.
On July 4, the Syrian Democratic Forces (SDF), supported by the US-led coalition, breached the Old City wall in Raqqa, advancing into its most heavily fortified enclave and overcoming the defenses of the terrorists.
The SDF launched the operation to reclaim Raqqa on June 6, more than three years after the Islamic State declared the city their capital.