I've noticed similar things myself. In Crysis 3 the framerate will plummet down to the 30s, and even 20s, in a few places. Typically where there are large moving metal objects. Then I turned all the settings to low, and set the resolution to 1080p. The framerate drop was nearly identical as very high settings and 1440p. This leads me to believe that there are issues with this driver. Don't forget, our 18.2.2 is using a Radeon RX 580 driver, because Apple can't be bothered with properly supporting Vega Pro 56/64 in Bootcamp. (Have you called Apple support about this? The more who complain, the more likely Apple will do something sooner rather than later).
Also, older games can have optimisation issues. I watched a youtube video of a guy playing Crysis 1 on a 21:9 34'' monitor (WAF channel), and the framerate would dip down into the 30s in certain areas. He was using a 1080ti and a 7700K. Just sayin'...
There is a quest in WOW Pandaria starting area, where to have to place a bomb in 4 different places to demolish the Horde's weapons.
Doing this quest on my Corsair One 7700K, 1080TI, 3440x1440P, 10/10 max, anti-alias max, 200% rendering, before I drop the bomb, the FPS usually hovers around 60-70, during the explosion, the FPS will drop to 25-35 for 2 seconds before it comes back up.
In iMac Pro, 5K resolution, 10/10, 100% rendering, no anti-alias, baseline FPS in Pandaria is around 44-50 for 80% of the time, during explosion, the FPS will drop to 15-25 for 2-3 seconds before it comes up.