The 10 FPS issue isn't linked to using drivers from the Oibaf PPA. Exactly the same thing can happen with 1:6.14.99~git20111219.aacbd629-0ubuntu2. It does not happen every time, so it can be confusing.
What happens is unhandled interrupts cause the kernel to disable the radeon IRQ. This is the IRQ used for waiting for vertical retrace, and I guess that wait times out after 0.1s. When things work properly, I can see radeon interrupt count increasing in /proc/interrupts while running glxgears and for a few moments afterwards. When things don't work properly, that count is 200000 and this appears in dmesg:
followed by a long backtrace and
[ 33.806034] irq 5: nobody cared (try booting with the "irqpoll" option)
This happened about 5 seconds after all the drm initialization messages. As you can see, the radeon irq handler was there. It just acted as if the interrupt was not from the video card. Earlier on there is:
[ 33.806980] handlers:
[ 33.807173] [<e8a57cc0>] radeon_driver_irq_handler_kms
[ 33.807180] Disabling IRQ #5
Since the interrupt is level triggered, I don't think it would be reasonable to use the noirqdebug kernel option to prevent the interrupt from being disabled. It would just keep interrupting and make everything slow or even unresponsive. I'll do some more investigation and report a bug eventually if it seems like there's a bug and not a hardware issue.
[ 24.047694] ACPI: PCI Interrupt Link [LNKA] enabled at IRQ 5
[ 24.047707] PCI: setting IRQ 5 as level-triggered
[ 24.047723] radeon 0000:01:00.0: PCI INT A -> Link[LNKA] -> GSI 5 (level, low) -> IRQ 5