In my opinion, this problem has not caused Mbappe performance to decline, he is still professional now and has contributed a lot to PSG and has score the most goals at PSG, so you are wrong if you think Mbappe has not performed optimally at PSG.
Mbappe must develop his career not only in the French league and maybe by joining Real Madrid Mbappe will be able to win the title he wants.
As we know, since joining PSG, Mbappe has not received any titles even though Mbappe is the best player like Messi and Neymar.
Depend on Mbappe still as professional player and not ignore yet for playing with PSG is not problem although still face bad communication with contract extend, bad result on last match when PSG defeating by Newcastle United is not mistake from Mbappe only but one team. I don't sure with PSG keep on consistency performance later when losing Mbappe due he has more contribution helped PSG winning domestic league every season. Not easy winning individual tittles for player only winning domestic league and Mbappe get difficult later if not winning Champion League with PSG I don't think he has chance with individual trophies such as Ballon D'Or.
More than five years Mbappe have been PSG players, but he still not winning most dreaming tittles as Champion League, I think he was loss patience if this season can't winning yet he will leave PSG.