Reducing Load by Randomly Dropping Expensive Rewrites
To reduce processing load, PageSpeed can be configured to optimize the most frequently fetched resources, leaving infrequently fetched resources alone. This is accomplished by randomly dropping expensive (CSS and image) rewrites. Frequently fetched resources will have a higher probability of being rewritten than infrequently fetched resources. Over time, frequently accessed resources will be optimized and cached so a page will be fully optimized. Infrequently accessed pages will be left unoptimized or partially optimized, saving CPU time and cache space.
pagespeed RewriteRandomDropPercentage Percent
This is a load-tuning parameter (integer between 0 and 100 inclusive) that controls the percentage of resource rewrites that are randomly dropped. Currently only CSS and image rewrites are randomly dropped, as they are the CPU intensive rewrite tasks. A value of 100 means all such rewrites are dropped and a value of 0 means no rewrites are dropped. A value of 75 means that 75% of image and CSS rewrites (selected at random) are dropped. Do not set this parameter to 100 in order prevent optimization of images and CSS files, it is more efficient to instead disable the image and/or CSS filters.
As an example, if the value is 90 then an image fetched only once will be optimized with 10% probability while an image fetched 50 times will be optimized with 99.65% probability (1 - 0.9^50). You may need to tune this parameter to find a value that provides the right load on your servers and still provides sufficient image and CSS optimization.
Note: Images within CSS files are not randomly dropped as this would lead to partially optimized CSS resources.