Solr Replication – High CPU Load – Frequent Commits-solr04032012

we are running a classified site and the latest ads which are coming to the system is indexed in solr write server in every three minutes. Then this index is replicated to solr salve servers, Over a period of time the solr index file size increased to 40 GB . Now we have noticed that when the replication happens the CPU usage is going very high for few seconds and coming back to normal. At this point of time the site just hangs and many read queries are not getting response with in the stipulated time. Is there any way to handle this situation, i know that frequent commits to solr slow down the performance.