I thought the issues had returned for my ISP, but turns out a Windows Update caused the "service host network service" to have high network usage via "delivery optimization" in Windows 10.

Went to check my retainers and noticed my ping was above 110ms, so I did a traceroute and it showed much higher ms on nodes well before NTT, so that told me something else was afoot. Then I opened Task Manager, and saw the culprit was indeed a Windows program called "service host network service" was gobbling up way more bandwidth than necessary.

So, after some YouTube searches, I used the command prompt "sfc /scannow" and that cleared up some files that must've been corrupted during the recent Windows Patch Tuesday. Took a few minutes to run, so some patience was required.

One restart later, and my latency's back to normal for FFXIV (around 59 to 64 ms).

Not sure if that'll help anyone else here, but it never hurts to check your PC to see if any Windows Updates of late messed with something. Chances are, knowing Microsoft, that's always a possibility.