Basic rendering incredibly slow with Sapphire RX 580

[Kaitain]

Just wondering if anyone has had a similar issue to what I'm seeing at the moment.

I'm setting up a new machine that has a Gigabyte 390m Gaming, an i7-9700K and a Sapphire RX 580. What I'm noticing is that when I take the HDMI output from the RX 580 rather than the CPU's UHD 630 output, the BIOS screen rendering is painfully slow, as is user input responsiveness. This slowness also continues into the Clover menus when setting up a new install, although, when I installed Linux Mint on one of my HDs, the screen rendering and responsiveness seemed perfectly snappy after booting into that OS.

Anybody know what might be going on? Should I be setting the BIOS to a particular setting to address this problem, or might it be an actual hardware issue? (Is there some diagnostic test that can be carried out?)

 

[Kaitain]

This whole thing is very weird. Sometimes I get no output at all from the Radeon card and need to fall back on the mobo's HDMI output. And sometimes THAT doesn't work unless I remove power from the card, i.e. I can be in a situation where I have the card plugged in but on boot NONE of the graphics outputs give me any signal (none of the four outputs on the RX 580, and not the mobo HDMI output either).

This is really irritating me. Whether various outputs work or not seems to be dependent on what the state of the last Clover boot was. Why should that be? Shouldn't a cold boot have no memory of the previous state of the machine? I have fast boot set to be disabled, but a lot of the time the mobo's initial splash screen appearance (or non-appearance) feels like it's connected with the setup of my Hackintosh build. e.g I've made changes with MultiBeast and this seems to kill the mobo splash screen from appearing. Ditto the speed of rendering. Anyone know what's going on?

I'd like to set the iGPU to "disabled" in BIOS, but at the moment I'm scared that this could leave me in a situation where I have no usable output at all.