Update:

Rebooted my PC. Windows automatically repaired a portion of C:. Open areas seemed to load faster, but I queue for MSQ Roulette. I load into Castrum Meridianum, see my whole party, the Search Light interactables, and the mobs. The other tank only pulls from the search lights. After the first cutscene, the party's at the first boss...and I barely have time to click "Teleport into area" before I'm hit with the next cutscene. Nothing loads to be targetted until the first Livia battle and the second phase is over before I come out of the cutscene between.
Afterwards, I fly to FATEs in the new areas, and the mobs aren't loaded in to target until a minute after I arrive, roughly 10:30a CST today (Wed).

Not sure what else I can do. Drivers appear to all be updated. All Win10 updates installed. I've reinstalled. I've put all my graphics settings to as minimal as possible. I no longer have a mobile hotspot to test if it's my home's wifi. I can't switch to a wired internet connection. I don't have the funds to acquire a SSD or new PC with one.