The Mishna Verura (494 sk 12) says one of the reasons for eating Milchigs on Shavuos
is because after Matan Torah, they couldn’t eat meat, because it was too time consuming.
They would need to shecht, remove the chailev , rinse ,salt and cook in a new pot.
They chose instead milchig.
What's pshat ?
Matan Torah was on Shabbos and couldn’t do all this because of issur Shabbos and not because
of takes too much time
This question was answered by the Chofetz Chaim himself.
(There exists today, a copy of his hand written answer.)
He answered that the reason he wrote, “they didn’t eat meat
because of lack of time” rather than because of issur
Shabbos, was, he wanted to give reason even according
to the Pirke d’Reb Eliezer who holds that the Torah was
given on Friday.*
*According to the Bavli , Hakol Modim Shebeshabbos Nitna Torah.