Game Performance halfed when using UEFI + Above 4G Decoding on MSI B450 Tomahawk with Vega 56

I've noticed a drastic performance regression if I enable UEFI Boot together with the Above 4G Decoding / Cryptocurrency Mining option of my MSI B450 Tomahawk (BIOS: 7C02v1D from 2019-11-11). GPU is a Vega 56 with Kernel 5.5.1, Mesa 20.1-git on openSUSE Tumbleweed. The issue is not Linux-specific though, as I get also a performance regression (and a Code 10 error from the LAN NIC) on Windows with these settings, albeit the performance loss is not that drastic. As benchmark I used the build-in benchmark of Company of Heroes 2 (Linux native) but it should also show in any other game.

Here are some numbers on different settings tested:

1. CSM + Above 4G Decoding enabled: 50 fps
2. UEFI + Above 4G Decoding enabled: 21 fps
3. UEFI + Above 4G Decoding disabled: 49 fps
4. CSM + Above 4G Decoding disabled: 51 fps

Is this a known problem with Large BAR support on that particular motherboard or B450 or AGESA in general? Is it expected behaviour?

Here is the output of inxi:

System: Host: localhost Kernel: 5.5.1-custom-2 x86_64 bits: 64 Desktop: KDE Plasma 5.17.5 Distro: openSUSE Tumbleweed 20200201 Machine: Type: Desktop Mobo: Micro-Star model: B450 TOMAHAWK (MS-7C02) v: 1.0 serial: UEFI: American Megatrends v: 1.D0 date: 11/07/2019 CPU: Topology: 6-Core model: AMD Ryzen 5 2600 bits: 64 type: MT MCP L2 cache: 3072 KiB Speed: 3189 MHz min/max: 1550/3400 MHz Core speeds (MHz): 1: 2754 2: 2651 3: 2719 4: 3013 5: 2351 6: 3435 7: 2325 8: 2314 9: 3729 10: 3731 11: 2379 12: 3362 Graphics: Device-1: Advanced Micro Devices [AMD/ATI] Vega 10 XL/XT [Radeon RX Vega 56/64] driver: amdgpu v: kernel Display: x11 server: X.org 1.20.7 driver: amdgpu FAILED: ati unloaded: fbdev,modesetting,vesa resolution: OpenGL: renderer: Radeon RX Vega (VEGA10 DRM 3.36.0 5.5.1-custom-2 LLVM 9.0.1) v: 4.6 Mesa 20.0.0-devel Audio: Device-1: Advanced Micro Devices [AMD/ATI] Vega 10 HDMI Audio [Radeon Vega 56/64] driver: snd_hda_intel Device-2: Advanced Micro Devices [AMD] Family 17h HD Audio driver: snd_hda_intel Sound Server: ALSA v: k5.5.1-custom-2 Network: Device-1: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet driver: r8169 IF: enp34s0 state: up speed: 1000 Mbps duplex: full mac: 00:d8:61:56:d3:9e Drives: Local Storage: total: 953.87 GiB used: 54.25 GiB (5.7%) ID-1: /dev/nvme0n1 model: SPCC M.2 PCIe SSD size: 953.87 GiB RAID: Hardware-1: Advanced Micro Devices [AMD] Device driver: N/A Partition: ID-1: / size: 937.41 GiB used: 54.24 GiB (5.8%) fs: ext4 dev: /dev/nvme0n1p2