@jsstaedtler
Unless you ISP did something funny, you should have 1500 on all interfaces. Nothing in this world should enforce you to go lower than 1400, and nothing out of this world should push you below 1300. (That is not to say it can't, just shouldn't).
In my case, I have had a 1400 set on one ethernet interface on mikrotik. That interface wasn't in use, but mikrotik sets the bridge MTU to the lowest MTU of its member interfaces. So effectively, all ports had 1400.
My PC would guess correctly, that something isn't right, and would send smaller packets, but some websites would still try to respond with packets over 1400. And when mikrotik send ICMP "too big", remote site ignores that, and just retransmits TCP packets until it gets bored.
TLDR: Check if all ports have 1500 first.