I'd think it has to do with the game's optimization, which is most likely close to nonexistent. Back in 2013-2014 they did a performance optimization patch but after WFA the performance has gone down a fair bit pretty much negating the optimization patch.
This game doesn't run well even on high end machines, at least not as well as it should from what I've heard of. For example, with my setup (not necessarily a high end one but proves the point):
i5 3570k OCed to 4,4 GHz
GTX 680 2GB of VRAM (OCed, core +145 MHz, memory +415 MHz)
8 GB DDR3 @ 1600 MHz
BF4 @ Ultra settings using 1080p resolution (but AA toned down to 2x and AA post @ medium) mostly at 60-100 FPS - nowadays BF4 is optimized pretty well I believe.
Anno 2205 at 1080p (high settings with AA off, still an being an unoptimized game but 3 years newer than CoH 2) at 40-60 FPS. However suffers from a memory leak and the performance goes down after 1-3 hours to an unplayable level.
I don't remember the exact CPU or GPU usage numbers of CoH 2 but I believe it doesn't use more than ~1/4 of my RAM. The game starts very smooth (without Vsync 80-100 FPS) but over time the performance drops drastically and in long games where the terrain is full of craters and stuff the FPS can drop as low as 20-40 even when there's no action at all. And CoH 2 doesn't even raise my CPU or GPU temps above 60 so it's not even near full load for either one. Sounds like a bad optimization and / or a memory leak to me. I don't expect my rig to run it at 60-100 FPS all the time but the performance is way worse than it should be.
Would be interesting to hear other players' rigs and performances for comparison