shifting ph or other factors that can cause a fluctuating growth rate can cause waviness in leaves or contortion. Early leaves often do funky things on their own, too. Since your newer 3-fingered leaves look fine in this regard, i woud not worry. The 3-fingers aren't always blemish-free, either. Assumes all other factors 'good enough.'
growth rates are relative... soil grows tend to be on the slower side of spectrum. The first couple weeks are boring but will double in size as a faster rate over time. Think of how much plants grow after going into flower phase.. 2-3x larger.. that's nothing more than continuing previous vege growth for 3+ weeks before abating. it's not "caused" by the flip. Cessation of vege growth (stems elongation + leaf growth) is caused by the flip to flower for photoperiods.
in soilless i'd expect that in 7-8 days from sprout... just a guess. You got a little more stretch than i go for, but looks like you adjusted by the time it mattered - the internode at top.. a little stretch between cotyledons and single-finger leaf is no big deal. use those resulting internode lengths as a guide for light intensity.
