The current generation of queries that supply a Filter will result in a root level filter. For example, if you had:

query.Query(q => q.Number > 10).Where(q => q.State == "WA");

then this generates something along the lines of

{
  "query" : {
    "range" : {
      "Number" : {
        "gt" : 10
      }
    }
  },
  "filter" : {
    "term" : {
      "State" : "WA"
    }
  }
}

Note: it's not really useful to score a range query as it either matches or not, but that's not the point.

However, using the root-level filter is actually using something called Post Filter, which is a post processing filter. This is useful when you want to pass certain hits to facets/aggregations, but then you need to prune them from being returned within the overall response (a rare need). What you actually want is a filtered query that itself contains a filter and/or a query.

The above should become:

{
  "query" : {
    "filtered" : {
      "query" : {
        "Number" : {
          "range" : {
            "gt" : 10
          }
        }
      },
      "filter" : {
        "term" : {
          "State" : "WA"
        }
      }
    }
  }
}

This will perform the filter before any query (or hits being returned), which is when you want it to happen.