How to improve Solr search performance

[李银松]

I'm testing solr search performance using LoadRunner
the index contains 5M+ docs , about 10.7GB large.
CPU:3.2GHz*2 RAM16GB
The result si dispirited
max:19s min 1.5s avg 11.7s
But the QTime is around 1s
(simple query without facet or mlt,just fetch the top 50 IDs)
So it seems that XMLWriter is the bottleneck
Am I right? Or is there anything i can do to improve the performance?