When PageSpeed attempts to rewrite an uncached (or expired) resource, by default it will wait for up to 10ms per flush window to finish and return the optimized resource if it's available. If optimization has not completed within that time the original (unoptimized) resource is returned and the optimizer is moved to the background for future requests. The following directive can be applied to change the deadline. Increasing this value will increase page latency, but might reduce load time (for instance on a bandwidth-constrained link where it's worth waiting for image compression to complete). Note that a value less than zero will cause PageSpeed to wait indefinitely.
pagespeed RewriteDeadlinePerFlushMs deadline_value_in_milliseconds