DataImportHandler is usually a good chance to make considerations about performance and tuning of a Solr installation. For example, while indexing we can suffer through problems that are mostly dependent on memory usage (of both the index and the documents to be posted), merge time, and optimization time.
These kinds of problems can be reduced by using the
omitNorms="true" parameter and studying a good value for the
mergeFactor parameter, both in
schema.xml. Merging is critical because it introduces an overhead, and it's easy to consider the same problem with optimization.
A good list of the factors in which we are interested for understanding the performance can be assembled from the following two wiki ...