The grayed out portion (the gap) is time that Salesforce spends doing stuff that you don't have control over. This could include allocating/freeing memory, waiting for a lock to be removed, initializing a DB connection, etc. Since this is the timeline for the execute method, the methods described in the white-paper for async processing shouldn't apply here.
Is this reproducible every time, or does that gap vary? Since it is a 5 second gap, I would guess that Salesforce is waiting for something. I wouldn't be surprised if it took 5 seconds to wait for records to be unlocked if there is another process going on. It could also depend on the current load of the instance. Running at 3am on Saturday would probably have smaller gaps than running at 10am on a Wednesday.
That timeline is completely generated from the apex log. There have been previous bugs with how the developer console shows the start and end of those gaps. It could be worth it to look at the raw log and see if there is indeed 5 seconds between those two events, and if there are any other event lines in-between.