Just chiming in to say I've got the same problem. Never had it before Stormblood but it's been around since the first day of early access. Happened a couple times in dungeons so I've started closing my client and re-launching it before queuing up since it normally takes a couple hours before the FPS gets awful (and then the inevitable crash).

My PC has an AMD card so I tried playing on my laptop that has a fine nvidia card (for a high-end laptop, anyway) but got the exact same issue after a few hours, so it definitely doesn't appear to be related to that :/