I have the same issue. Just sitting in my base I get around 18 FPS. Outside it can go up to 22-28 FPS.
Horde nights, or inside large POIs like Dishong Tower it drops to 3-7 FPS and gets really choppy.
I have updates/anti-virus/etc. disabled while playing and monitoring the system usage while playing I see the average GPU load goes no higher than 55% and it's temp sits around 50-60 degrees C. For comparison, running the 3DMark DX11 benchmark sees the GPU hitting 80 degrees C and 100% utilization and the recorded frame rate in the benchmark was between 36-51 FPS.
Overall CPU utilization doesn't get above 35%. The highest utilization of a single core is no higher than 75%.
Memory and disk drive usage are low to negligable.
Specs
CPU
2 x Intel Xeon Processor E5-2650
- Clock frequency: 2,778 MHz (2,000 MHz)
- Physical / logical processors: 2 / 32
- # of cores: 8
GPU
AMD Radeon RX 480
- Memory: 8,192 MB
- Clock frequency: 1,310 MHz
- Memory clock frequency: 2,100 MHz
- Average temperature 78 °C
RAM
32 GB Kingston HyperX (2 x 16GB matched sticks in separate channels)
Game Drive
2 x 250GB Samsung EVO SSD configured RAID0
OS
64-bit Windows 10 Pro (10.0.19041)
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My wife gets 100+ FPS in our base and 70-95 FPS on horde nights/dishong tower.
For comparison to my specs above, my wife's PC has the following specs...
Specs
CPU
1 x i5-4460 CPU @ 3.20GHz (4 CPUs), ~3.2GHz
GPU
NVIDIA GeForce GTX 960
- Display Memory: 8119 MB
- Dedicated Memory: 4053 MB
RAM
8 GB Generic (1 x 8GB)
Game Drive
1 x Seagate Barracuda 7200RPM HDD 1TB
OS
64-bit Windows 10 Pro (10.0.19041)
As you can see, her PC is far less powerful than mine. I don't understand what it is on mine that's making the game run so poorly.