Your right, I frequently look as inspect-delay as not being a real line given that it doesn't do much itself. I have an explanation of inspect-delay a bit further down: When tcp-request inspect-delay is present, it will hold the request until the rules in that block have the data they need to make a decision or until the specified delay is reached, whichever is first.

Basically if an ACL requires an http request (such as updating the http_req_rate counter) but the action is tcp-request it tells HAProxy to wait until it has a http request before it continues. If using an http-request track-* action its not required as HAProxy won't process http-request until it has an HTTP request to add to the counters.

Since HAProxy won't send a request to a backend in HTTP mode until it has a full request inspect-delay doesn't actually delay the request at all; just the processing phase for a bit (in TCP mode where it doesn't it gets a bit more complicated).