Question 240hz monitor: brightness drops with refresh rate?

Sep 29, 2020
So, I upgraded my Iiyama XB2380HS to an Asus vg279qm, having excellent reviews on reputable sites. Setting up the monitor at 120 or 240hz, with a DisplayPort cable, the brightness takes a steep nosedive when I raise the refresh rate. From "very bright" at 60hz (its rated 400cd/m²) it drops to a lower brightness than my old monitor (250cd/m²) at 240Hz. The display is in sRGB mode. What's happening?

Also, I don't see any difference between 60hz and 240hz in games or with desktop use. The mouse pointer still has visible ghosting. The game I mostly play only runs at 100fps max, but it should still be "better". ELMB sync is on. The display shows a big G-sync logo on startup and the specs say "G-sync compatible", but that just means it has no G-sync hardware and my GTX970 can't use it.
Overall the monitor is a pretty disappointing experience. The adjustable display stand doesn't go high enough, and I'm not even a tall person. The standard profiles are garbage, and the sRGB mode disables a lot of options. You don't get a manual with it, and the online manual barely describes the maze of available settings and what is compatible.