graphic: 0.472
x86: 0.192
device: 0.158
semantic: 0.133
performance: 0.103
architecture: 0.090
vnc: 0.057
peripherals: 0.054
virtual: 0.053
ppc: 0.049
boot: 0.048
register: 0.048
user-level: 0.043
mistranslation: 0.042
debug: 0.041
socket: 0.040
network: 0.033
permissions: 0.032
PID: 0.030
hypervisor: 0.025
files: 0.019
KVM: 0.018
risc-v: 0.015
VMM: 0.014
kernel: 0.013
TCG: 0.013
arm: 0.011
i386: 0.008
assembly: 0.007

Cirrus graphics results in monochrome colour depth at 640x480 resolution

Recently we upgraded to a distribution that bundled QEMU 4.2.0.  We were previously running on QEMU 3.0.0.  When booting Windows 10 VMs on x86_64, users experienced slow, monochrome graphics and the resolution was restricted to 640x480.  Reverting to the prior vgabios-cirrus.bin from the prior source tarball remediated the issue.

An example QEMU command line is below, if needed:
/bin/qemu-system-x86_64 -vnc 0.0.0.0:100 -device cirrus-vga,id=video0,bus=pci.0,addr=0x2  -machine pc-i440fx-4.2,accel=kvm,usb=off,dump-guest-core=off -cpu qemu64 -m 2048 -overcommit mem-lock=off -smp 1,sockets=1,cores=1,threads=1 -no-user-config -nodefaults -hda test.raw &

This seems to be the following SeaBIOS bug:
https://<email address hidden>/msg12271.html

Ah, great catch!  Yes, that does appear to be the issue+fix.  Is QEMU planning on integrating the next release of SeaBIOS or fixing this as a one-off for now?

Yes, the maintainer will likely get the submodule updated before the next release.

This issue is still marked as unresolved almost 6 months later. Has the submodule been updated and merged into QEMU yet?

The patch mentioned by Philippe ("vga: fix cirrus bios") has been included into QEMU via this commit here:
https://git.qemu.org/?p=qemu.git;a=commitdiff;h=de15df5ead400b7c3d0cf2
... which has been released as part of QEMU v5.1 already. Thus this issue should be fixed now.