Your first lap is faster because you aren’t fatigued.
This.
To expand, your first lap can be quite anaerobic (especially if you 'go for it, but still partly so even if trying to be steady) - so quite speedy. But it’s not sustainable.
I think I’m Michael Phelps for 30-35m. Then I find out after 40 that I really am not.
Yup. Couple it with excitement/energy/motivation to get started.
And let’s not forget the stimulative effect of “OMG, that water’s cold!”