I apologize, I may have mislead you a bit. The U-Boot splash screen stuff does not work at all in the Dunfell branch; there is not a proper driver in the version of U-Boot used on that branch. We are investigating support for it on BSP 6 but at the moment it is not working.
As for the kernel logo, that should work with HDMI although I have not explicitly tried it on apalis-imx8. In most cases it seems that the graphics subsystem takes a while to stabilize and in my case the HDMI display does not even sync until later in the boot.
There are other splash screen utilities that run in Linux that we use for example in Torizon which allows yet another splash screen. For Torizon we use Plymouth.
I don’t believe there is one enabled in the multimedia image but you certainly could do so but whether it would be any faster than just waiting for the standard GUI display to come up I don’t know.