it is in the README:
~$ less /usr/share/doc/NVIDIA_GLX-1.0/README.txt
Option "UseEvents" "boolean"
Enables the use of system events in some cases when the X driver is
waiting for the hardware. The X driver can briefly spin through a tight
loop when waiting for the hardware. With this option the X driver instead
sets an event handler and waits for the hardware through the 'poll()'
system call. Default: the use of the events is disabled.
too bad it does not do it for you. For me, this option changes things a lot!