I reached out to the Radeon Linux driver maintainers and they indicated that this patch improves Radeon RX 5700XT performance: 46203a508f64b4bfa150a9d25eab1dc891e7e650
This change only appears in 5.5 kernels. I can confirm that kernel 5.5rc7 (latest on https://kernel.ubuntu.com/ at time of writing) can improve OpenCL performance by more than 10x.
Ubuntu 18.04.3 doesn't include kernel 5.5 yet, so to get this improvement, you'll need to install it yourself. Go to the Ubuntu kernel page, choose the 5.5 kernel version you want, find your architecture, choose low-latency or generic, and download linux-headers-..._all.deb, linux-headers-..._<arch>.deb, linux-image-..._<arch>.deb, linux-modules-..._<arch>.deb
Run:
sudo dpkg -i linux-headers-..._all.deb sudo dpkg -i linux-headers-..._<arch>.deb sudo dpkg -i linux-image-..._<arch>.deb sudo dpkg -i linux-modules-..._<arch>.deb
Reboot to use the new kernel.
