Very bad search performance with group=true

[de...@geschan.de]

Hi,

I'm indexing pdf documents to use full text search with solr.
To get the number of the page where the result was found, I save every  
page separately and group the results with a field called doc_id.
(See this topic:  
http://mail-archives.apache.org/mod_mbox/lucene-solr-user/201303.mbox/%3C1362242815.4092.140661199082425.338ED62D@webmail.messagingengine.com%3E  
)

This works fine if I search in a single document, but if I search over  
the whole database  for a term, the results are really really slow,  
especially if group.limit is above 10. I indexed about 150.000 pages  
for now, but in the end it will be more than 1.000.000 pages.

How can I improve search performance?

I'm using this configuration:

<requestHandler name="/search/fulltext" class="solr.SearchHandler">
      <lst name="defaults">
	   <str name="echoParams">explicit</str>

	   <str name="wt">json</str>
	   <str name="indent">true</str>
	   <str name="df">text</str>

	   <!-- Query settings -->
	   <str name="defType">edismax</str>
	   <str name="qf">
	          id^10.0 ean^10.0
		  title^10.0 subtitle^10.0 original_title^5.0
		  content^3.0
		  content_en^3.0
		  content_fr^3.0
		  content_de^3.0
		  content_it^3.0
		  content_es^3.0
		  keyword^5.0 text^0.5
                   author^2.0 editor^1.0
		  publisher^3.0 category^1.0 series^5.0 information^1.0
	   </str>
	   <str name="mm">100%</str>
	   <str name="q.alt">*:*</str>
	   <str name="rows">10</str>
	   <str name="fl">id, title, subtitle, original_title, author,  
editor, publisher, category, series, score</str>
	   <bool name="group">true</bool>
	   <str name="group.field">doc_id</str>
	   <int name="group.limit">20</int>
	   <bool name="hl">true</bool>
	   <str name="hl.fl">content_*</str>
	   <str name="hl.alternateField">content</str>
	   <bool name="hl.requireFieldMatch">true</bool>
	   <str name="hl.simple.pre"><![CDATA[<strong>]]></str>
	   <str name="hl.simple.post"><![CDATA[</strong>]]></str>
      </lst>
</requestHandler>

THX for your help.

- Gesh