Snappy software requires the design of the system to be in sync with the engineering. Also the team needs to be aligned on snappiness being a priority.

Product designers not understanding how adding certain UI elements, animations or requesting certain features is the main cause from my experience. The fastest way to speed things up in certain cases is to just remove some elements.

I would expect the situation to get worse rather than better if LLMs can deliver any feature a product designer wants without limitation and if the product designer doesn't care about "snappiness"