40% performance loss in Mesa with Linux 5.0

Mesa's RADV performance has dropped nearly 40% across multiple applications since upgrading my HWE kernel to 5.0.0-23.

Reverting to the 4.18 kernel fixed the issue. The problem was still present when testing kernel 5.2.8 from the Ubuntu mainline PPA installed via UKUU. The latest LTS kernel release (4.19.66) worked perfectly.

My hardware:
AMD Radeon RX 570
AMD Ryzen 7 1700X
Asrock B450M Pro4 motherboard

Tested Ubuntu kernels:

4.18.0-25 (good)
5.0.0-23 (bugged)
5.0.0-25 (bugged)

Tested mainline kernels:

4.20.17 (good)
5.0.3 (bugged)
4.19.66 (good)
5.2.8 (bugged)

Tested versions of Mesa, ALL suffered performance loss under the bugged kernels:
19.0.2 from the repository
19.0.8 from the repository
19.1.2 stable from PPA
19.2-devel from PPA

Comparative performance testing was done through the benchmark tool of a native Vulkan game (Total War: Three Kingdoms), and in Steam titles using DXVK and Proton with the Steam framerate monitor.

Here is the output of sudo lshw -C:

  *-display
       description: VGA compatible controller
       product: Ellesmere [Radeon RX 470/480/570/570X/580/580X]
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 0
       bus info: pci@0000:23:00.0
       version: ef
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller bus_master cap_list rom
       configuration: driver=amdgpu latency=0
       resources: irq:46 memory:e0000000-efffffff memory:f0000000-f01fffff ioport:e000(size=256) memory:fe900000-fe93ffff memory:c0000-dffff

Thanks for taking the time to look into this.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: linux-image-5.0.0-23-generic 5.0.0-23.24~18.04.1
ProcVersionSignature: Ubuntu 5.0.0-23.24~18.04.1-generic 5.0.15
Uname: Linux 5.0.0-23-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.7
Architecture: amd64
CurrentDesktop: KDE
Date: Sun Aug 11 04:15:28 2019
InstallationDate: Installed on 2019-02-01 (191 days ago)
InstallationMedia: Kubuntu 18.04.2 LTS "Bionic Beaver" - Beta amd64 (20190129)
SourcePackage: linux-signed-hwe
UpgradeStatus: No upgrade log present (probably fresh install)