I am experiencing the very same issue. And it all started 2 to 4 weeks ago.

FPS constantly drop down to 10-25 FPS from 80-100 FPS, before recovering again. This happens every 5 to 10 seconds and lasts for as long. Almost as if the game is loading the the patch of the map you are currently in and when its done your FPS go back to normal before the cycle start all over again.

It happens across all zones. Even in almost empty PvE zones with no other player or NPC in sight.

I noticed it most when I almost always noticed it when I was on the move / riding. But that might not be a very objective observation as low FPS are far more obvious and annoying if you are moving (fast) compared to standing still.

I am not sure if water plays a role in this. I have had the FPS drops occoring everywhere, even in places where there was no water in sight - even in the middle of Alikr Desert. However, standing at the Artaeum Wayshrine and looking towards that pool behind it makes my FPS drop from 100 down to 50-60 with water settings on Medium. Turning away from the pool lets the FPS recover to 100. Maybe that's a normal FPS drop for water pools. Maybe it is not. Whatever the case, there seems to be more going on as 50-60 FPS is still "enjoyable" while the 10-25 FPS I constantly get is causing me headaches.

My graphics card is a NVIDIA GeForce GTX 1660 Ti up-to-date. And lowering the graphics settings ingame had no noticeable effect on the FPS - though I have not tested it below medium settings so far, as 2 to 4 weeks ago High to Ultra settings were no issue - Cyrodiil excluded obviously.

This fps drop issue just makes the game unplayable after pvping for an hour(144fps to 19-25fps). If you watch Feng's stream everyone including him has to restart they're game every hour because of this issue. Its ridiculous how "Memory Improvements" have made the game perform the worse its ever been.