I am relatively new to SOLR. We are using SOLR 6.1.0.1 with Sitecore 8.1 R3. There are roughly 50K items (content and files) being indexed. We are being plagued by poor throughput (through put is about 77%), general slowness, and errors in logs such as consecutive full GC and errors such as "Error opening new searcher. exceeded limit of maxWarmingSearchers=2". The SOLR server has 8 cores and 16GB of memory. We are running 64bit version of Java in SOLR. I have increased the heap space to 4GB. I am not seeing Out of Memory exceptions in the SOLR logs. SOLR is installed on a single server. We are not using sharding, SOLR Cloud or an other distributed type SOLR instance.

Any Advice would be much appreciated