There are a few things that could be impacting the search times but off the top of my head i would say that what you are seeing may be a result of your index exceeding the available disk cache. 8gb for OS, other processes and disk cache may not be enough and thus you may be hitting the HDD's more than you think for the large index scenario. Looking at iostat for the disk(s) in question may be helpful.
That being said, it could be something else as well. In my use case I don't really have parent/child in my index so I can't say if that is a factor or not. You may be able rule it in or out just by querying a single type at a time though and seeing if the times are drastically different.
This document may be helpful - https://www.elastic.co/guide/en/elasticsearch/guide/current/parent-child-performance.html