Very Low Performance With LogStash
Try reducing your workers in your elasticsearch output.
From a blog post by elastic on Logstash optimization(emphasis mine):
...modify configuration variables on your output (most notably the “workers” option on the Elasticsearch output which will probably be best at the number of cores your machine has)...
Another thing brought up by the article is the fact that the bottleneck may be your elasticsearch. 2GB of memory is tiny for an elasticsearch node and the problem may lie in a resource choked elasticsearch rather than a misconfigured logstash.