Did pacman -Syu, now there's a really long delay after pressing the Fn-related keyboard keys (screen lock, projector, brightness, etc.) on my laptop. Testing using acpi_listen, it can take over 25 seconds before the key registers after pressing it.
Sometimes (maybe after sleeping and resuming?) the delay time gets shorter, but in general they still seem not very responsive.
It seems that the issue might be related to acpid_video0, which I was using to control the backlight brightness. Switching to intel_backlight seems to be more responsive at the moment. I'm still confused because earlier it seemed that it was the ACPI events that were registering slowly so it shouldn't matter how I'm actually handling them.