Poor unless you have a macOS-supported GPU to put through PCI passthrough. Otherwise, macOS will not use graphical acceleration and the system UI will be very glitchy (e.g. severe ghosting) and slow to respond.

Is the non-GPU passthrough performance bad enough to impact non-graphical applications? Is the whole UI just slow without it?

Well there are 3 realistic options here:

Intel igpus

Nvidia

AmdATI

Linus Tech Tips did a review of this method, including a performance comparison: https://www.youtube.com/watch?v=ATnpEOo3GJA

Ever boot your Mac in safe mode, where you can see the screen re-drawing because acceleration is off? It's like that.

It looks like this:

https://megous.com/dl/tmp/nbfcbjbdceowrvuepvoj.mp4

Software rendering. Not terrible. Ivy Bridge.

I did not even consider about messing with GPU passthrough.

With or without GPU passthrough?

Without

I'm not using this script, but it's reasonable. Good enough for Safari/web testing, or even running Xcode/iPhone emulator.

That's good to hear.

Is this a viable option to have running on a Linux box so that you can use it as a CI for testing iOS apps?

Or to be specific, what kind of specs, i.e. the amount of RAM would be needed to have a reasonable performance?

Thanks

For both iOS simulator and Safari, the lack of GPU acceleration can be a problem, depending on what/how you're trying to test, and also depending on OS version. I believe Xcode 11 requires a Metal-capable GPU in order to even run. As someone else has mentioned, you can however use IOMMU/PCIe passthrough with certain graphics cards to get acceleration inside the VM.