I suspect this is caused by the HardDPMS option, which is enabled by default now. When active, this limits the frame rate to 1 fps when DPMS kicks in because otherwise, applications would start rendering as fast as possible.
However, that limit shouldn’t be applied for displayless setups like Bumblebee. I filed internal bug 2775447 to track fixing that.
In the meantime, you should be able to work around the problem by disabling DPMS in the Bumblebee X session, either by disabling the HardDPMS option in xorg.conf or by running “xset s off -dpms” in the sessions setup.