Just want to share my experience with FPS-Drops and how i solved it (at least the biggest drops).
Ryzen 5 2600X
16 GB RAM DDR4 3200
Radeon R9 390 ---> changed to Radeon RX 5700
With my old R9 390, i had no huge FPS drops and the game was running around 120 fps average at FullHD (with a range from 90 - 160 and more fps, depending on Map). I limited the FPS to 120 in the settings to just make them not going up or down too much.
When i changed my graphics to RX 5700 (with a clean deinstallation of the old driver and new installation of the latest card drivers), i suddenly had very big FPS drops and an overall bad performance in the game.
Of course, i was wondering how this could be possible with a better graphic card! Was searching for the problem and found it in the power plan profile for Windows.
When the Ryzen came out, the standard Win10 power plan caused the CPU to go into power savings (performance throttled to 10%) and not waking it up back to full performance when needed.
AMD implemented their own power plan into their Chipset-Driver to avoid this (power plan called "Balanced for Ryzen"). This power plan is setting the lowest CPU performance to 90%.
In my case, with the deinstallation of the card driver, the Ryzen power plan disappeared too, so i re-installed the Chipset driver.
Ryzen power plan was back again and the performance in Ironsight was way better again. It looks, like the game is still sensible to this specific problem.
Would recommend to set the power plan to high performance or at least change the CPU power saving options in the standard "Balanced" plan when you're playing Ironsight ... regardless of the used CPU, Intel or AMD.