LED lights delayed when turned on
It's not the wiring.  It's the LEDs themselves. 

LEDs have a characteristic delay when turning them on which may be more than you're used to from a lifetime of incandescent bulbs. 

Swap the LEDs from one position to another, and the problem should move with the bulb.  Replace the LED with the equivalent incandescent (temporarily) and it should go away entirely. 

Unfortunately, home LEDs are still relatively new and they suffer from both this characteristic delay --- which may vary from brand to brand -- and manufacturing variation --- where bulbs from the same manufacturer may vary, even within the same batch. 

The only way to avoid these issues is to closely watch online and professional reviews (from trusted sources) to pick the better-performing brands; return any product to the store that is an "outlier" in the same room -- or, wait another few years for LEDs to mature (I'm serious) and just live with it for now.