Why has Spain’s economy been so historically weak since the 18th century?

I know this veers into history territory but the Spanish empire — at its peak — was flush with Aztec and Incan gold. It was the largest empire in the world and was one of two powers, the other being Portugal, to have a city in east asia under their control which granted them access to China and Chinese goods.

And yet by the time 18th century comes around it’s already falling behind. By the 19th and well into the early 20th it’s not much better. The first 30 years of the 20th century in Spain are chaotic and bloody, then we get Franco and after Franco the current Republican government. But all along the way they still suffer in comparison to other western European nations.

Is that a fair assessment?