Reduce the number of addons, especially database intensive ones such as Harvest Map and Master Merchant with the below tip.
Use Circonians Addon Selector to select a addon profile for your mules, main char and resource gathering toons. Enable only addons that you really need for that character. For example, your mules do not need Master Merchant or Harvest Map.
Disable unneeded addons when in Cyrodiil. You can create a profile in Circonians Addon Selector to enable only essential addons and switch to it conveniently when in Cyrodiil. Do you really need Master Merchant or Harvest Map running there?
Use Votan’s Adaptive (Video-)Settings to reduce the View Distance on the fly depending on the FPS.
And avoid grouping in Cyrodiil unless you like 10-min loading screens – it’s a bug.
Finally, the 99% cause of low FPS is due to the game’s single-core oriented programming. You can verify this by monitoring your GPU Utilization in MSI Afterburner with the game running. If it does not hit 90% and above, your CPU is the bottleneck. Upgrading your graphic card will not generally offer any improvements, but upgrading to a higher single-threaded performance CPU will.
Here’s a current list of the latest CPUs sorted by highest single-thread performance. Don’t expect a 50% improvement in FPS. Don’t expect 60fps smooth framerate in Cyrodiil when there’s lots of players around even with a 9900k 2080ti (yes, I’ve tested it).
Lowering the video mode to Medium or Low will only help if your Graphics Card is not the bottle neck.
Even World of Warcraft suffers from this problem. Here’s a video demonstrating the problem