When I run Arch on my laptop without AC power I get the error "hda: irq probe failed" and the system is bloody slow. For example, it takes five seconds for 'password' to appear when I type my login. A good five minutes to start X/KDE. When I reconnect power it is still slow, unless I boot with the power on.
This is very annoying because I can only use Windows when I'm running on the battery.
For me this happens when my battery is almost empty and the APM system starts "yelling" IRQ's and flooding the kernel with it.
Since I can't read any more information from your post, maybe it's something similar. Maybe the IRQ setup is wrong, the APM setup, the ACPI setup .... try turning APM and/or ACPI off or pass different parameters for whatever you have. There are APM and/or ACPI HOWTOs out there - also a Battery-powered-HOWTO