Had .5 of usual glxgears fps.
Tried everything I could think of without success.
Same kernel you have, no problems up/downgrading drivers here.
Finally went to bios and told it to write ESCD on exit.
And why not. Success. glxgears fps back to normal!
Did not add/remove devices or play with bios setting
before success. However played a lot with kernel
configuration parameters for recompiles
(tweaking aiming for perfection + some new framebuffer stuff).
Maybe this modified something requiring escd
to refresh or something ?
Not even sure what escd is.
Maybe it's the cmos ram or some other addon voodoo.
Anyway joy here and wish you same easy fix.