SOLR is slow at first facet query but quite fast f

I'm trying to figure out why my SOLR (4.1 )instance is extremely slow for facet queries. The index has approximately 200M documents and the server has 64GB RAM.

My query looks like this:

q=CampaignId:1462%0ASourceDateUtc:[2014-01-01T00:00:00.000Z TO 2014-01-30T00:00:00.000Z]
&wt=xml&indent=true&rows=0
&facet=true&facet.field=UserName&facet.mincount=10&facet.method=fc

It would take like 6 minutes for the first hit but when the result comes back, I search again with the same query or slightly change the range in SourceDateUtc, it runs quite fast.

Here is my solrconfig.xml (query section)

<query>
  <!-- Cache used by SolrIndexSearcher for filters (DocSets),
         unordered sets of *all* documents that match a query.
         When a new searcher is opened, its caches may be prepopulated
         or "autowarmed" using data from caches in the old searcher.
         autowarmCount is the number of items to prepopulate.  For LRUCache,
         the autowarmed items will be the most recently accessed items.
       Parameters:
         class - the SolrCache implementation (currently only LRUCache)
         size - the maximum number of entries in the cache
         initialSize - the initial capacity (number of entries) of
           the cache.  (seel java.util.HashMap)
         autowarmCount - the number of entries to prepopulate from
           and old cache.

    <filterCache
      class="solr.LRUCache"
      size="1024"
      initialSize="512"
      autowarmCount="0"/>-->

   <!-- queryResultCache caches results of searches - ordered lists of
         document ids (DocList) based on a query, a sort, and the range
         of documents requested.  -->
    <queryResultCache
      class="solr.LRUCache"
      size="10000"
      initialSize="512"
      autowarmCount="0"/>

  <!-- documentCache caches Lucene Document objects (the stored fields for each document).
       Since Lucene internal document ids are transient, this cache will not be autowarmed.  -->
    <documentCache
      class="solr.LRUCache"
      size="1024"
      initialSize="512"
      autowarmCount="0"/>

    <!-- Example of a generic cache.  These caches may be accessed by name
         through SolrIndexSearcher.getCache().cacheLookup(), and cacheInsert().
         The purpose is to enable easy caching of user/application level data.
         The regenerator argument should be specified as an implementation
         of solr.search.CacheRegenerator if autowarming is desired.  -->
    <!--
    <cache name="myUserCache"
      class="solr.LRUCache"
      size="4096"
      initialSize="1024"
      autowarmCount="1024"
      regenerator="org.mycompany.mypackage.MyRegenerator"
      />
    -->

    <!-- An optimization that attempts to use a filter to satisfy a search.
         If the requested sort does not include a score, then the filterCache
         will be checked for a filter matching the query.  If found, the filter
         will be used as the source of document ids, and then the sort will be
         applied to that.
      -->
    <useFilterForSortedQuery>true</useFilterForSortedQuery>

    <!-- An optimization for use with the queryResultCache.  When a search
         is requested, a superset of the requested number of document ids
         are collected.  For example, of a search for a particular query
         requests matching documents 10 through 19, and queryWindowSize is 50,
         then documents 0 through 50 will be collected and cached. Any further
         requests in that range can be satisfied via the cache.
    -->
    <queryResultWindowSize>100</queryResultWindowSize>

    <!-- This entry enables an int hash representation for filters (DocSets)
         when the number of items in the set is less than maxSize. For smaller
         sets, this representation is more memory efficient, more efficient to
         iterate over, and faster to take intersections.
     -->
    <HashDocSet maxSize="3000" loadFactor="0.75"/>


    <!-- boolToFilterOptimizer converts boolean clauses with zero boost
         cached filters if the number of docs selected by the clause exceeds the
         threshold (represented as a fraction of the total index)
    -->
    <boolTofilterOptimizer enabled="true" cacheSize="32" threshold=".05"/>

    <!-- Lazy field loading will attempt to read only parts of documents on disk that are
         requested.  Enabling should be faster if you aren't retrieving all stored fields.
    -->
    <enableLazyFieldLoading>false</enableLazyFieldLoading>

    <!-- Use Cold Searcher

         If a search request comes in and there is no current
         registered searcher, then immediately register the still
         warming searcher and use it.  If "false" then all requests
         will block until the first searcher is done warming.
    -->
    <useColdSearcher>true</useColdSearcher>

</query>

I also tried to enable the filterCache but it doesn't help.

Thanks.

Likely a warm-up issue. Warm-up field cache( facet.method=fc) is very important for solr to work effectively. In case you haven't configure the warmup queries, please consider to add the facet query as in your example, to newsearcher and firstsearcher section in solrconfig.xml.

http://wiki.apache.org/solr/SolrConfigXml#A.22Query.22_Related_Event_Listeners

<listener event="firstSearcher" class="solr.QuerySenderListener">
      <arr name="queries">
        <lst> <str name="q">*:*</str>
              <str name="start">0</str>
              <str name="rows">10</str>
              <str name="facet">true</str>
              <str name="facet.field">UserName</str>
              <str name="facet.mincount">10</str>
              <str name="facet.method">fc</str>
        </lst>
      </arr>
</listener>

You may also want to turn off the useColdSearher