I have a 2002 GMC Envoy (4.2l v6).  83,000 miles.  The car has been bulletproof.  Other than a few maintenance issues, it's a great station car. We only put about 5k on it a year.

Recently, the car has been a bit slow to start.  It cranks fine, and the starter turns OK, but it takes about 15-20 seconds to catch.  It's almost as though it has to prime the fuel lines from the tank to the engine.  It's worth noting that this only happens with the first cold start of the day.  Any subsequent starts, and the engine catches within 5 seconds.

What do you think? Plugged fuel filter, fuel pump going bad, or something else?

I'd check filter for lots of dirt or moisture, air in the lines, and then fuel pump last.

Try turning the key to "on", then wait a few seconds before cranking it.  If the engine starts normally when you do this, I'd suspect that your fuel system is losing it's prime, or bleeding off pressure when it sits overnight.

When is the last time you replaced the plugs?

Possible fuel pressure regulator dropping fuel pressure.

On that car I would guess it's the fuel pump. Check valve is leaking and the pump is weak enough it doesn't prime instantly.

If not fuel related,  crank or cam sensor.