Historically, fiat is a paper representation of the physical gold. 

There are several neutral locations where you don’t have to fly yellow rocks across the world. Switzerland, UAE, Singapore, and etc. Even if countries didn’t trust those nations. 

There is a standard for metals in specific bullion vaults. So verifying would be minimal. Modern cargo plane could deliver 15 billion dollars worth of gold overnight from anywhere. 

So, I am not sure the slowest.

Today, Saudi Arabia takes Yuan. SA immediately go the Shanghai exchange use those Yuan and buy gold and have it delivered immediately.