Limiting the number of concurrent image optimizations
When optimizing images, PageSpeed can use significant CPU resources. As the results of the image optimization are cached, this is not ordinarily a concern once the cache is warm. But when PageSpeed is first installed, or when a corpus of new images is added to the server, PageSpeed needs to avoid having each process consume maximum CPU. To accomplish this, PageSpeed keeps a server-wide counter of active image optimizations. It avoids running more than
ImageMaxRewritesAtOnce image optimizations in parallel across all processes. The default value is 8. Override this in the configuration file to change this maximum.
Note: Limiting image optimization concurrency only applies to root and server level configuration.