If you check your squid forward (transparent or not) proxy log files you may found errors like those:
WARNING: All url_rewriter processes are busy.
WARNING: up to 6 pending requests queued
This is true if you use the directive “url_rewrite_program”, for example with SquidGuard.
In this case, squid tells you that it cannot spawn more helper processes to externally scan your requests in parallel, so it’s queuing your requests.
This is not a great problem, but you may be annoyed to see this stuff in your log files, or there are cases in which the default may be too low!
You may raise this limit with the parameter called url_rewrite_children.
To solve, add something like this to your squid.conf configuration file, and restart squid: