Long running search

Hello,

We have AEM 6.0 with SP1 and cq-6.0.0-hotfix-5354-1.0.0.zip installed(Oak 1.0.8). Size of repository ~200 GB and Lucene index 1.7 GB.

We are faced with a problem that the search through Content Finder or with QueryBuilder is performed for a long time. Even if we limit path to a folder with several assets, search can takes from 20 sec to 60 sec. Search via content finder by "test" keyword takes 17-90 sec and sometimes doesn't return any result.

I've added DEBUG logging for next packages:
org.apache.jackrabbit.oak.jcr.query
org.apache.jackrabbit.oak.query
org.apache.jackrabbit.commons.query.GQL
org.apache.jackrabbit.oak.plugins.index

Flowing the log files we can see that the search process is suspended at the class org.apache.jackrabbit.oak.plugins.index.lucene.LuceneIndex

09:04:48.050 org.apache.jackrabbit.oak.plugins.index.lucene.LuceneIndex loading the next 1600 entries for query :fulltext:test
09:04:48.070 org.apache.jackrabbit.oak.plugins.index.lucene.LuceneIndex ... took 20 ms
other threads work here...
09:05:18.487 org.apache.jackrabbit.oak.plugins.index.lucene.LuceneIndex loading the next 3200 entries for query :fulltext:test
09:05:18.651 org.apache.jackrabbit.oak.plugins.index.lucene.LuceneIndex ... took 164 ms

Time between execution start and execution end for this search request was 93 sec.
At the same time, search request that has no results, for example "qwerty" executes very fast (300-500 ms).

Please advise what might be causing of the slow search or how we can identify it?