Vertica INSERT speed highly depends on server load
You shouldn't be surprised that a load takes longer when there's more resource usage. However, there's probably a simple explanation for your poor performance. I would examine the following:


The INSERT with UNION ALLs is highly suspicious. Is the load more than 100MB? If yes, use COPY. 
Are the projections optimized for load?
Is the query profiled and no locks being used on the target table?
Most importantly, is the load local? From a client across a wide network? Test a load from a local file as a benchmark.


Your "1000 rows" isn't helpful because your row can be 10MB wide, or you could be loading JSON, loading LONG VARCHAR, etc. An actual load size is more valuable. On a typical DL380 Gen2, you should see around 350GB/HR/node.