Hi CPU utilization when highlighting mergecontiguous=true

A user's list thread indicates that Solr 6.3 has very high CPU utilization with highlighting and mergecontiguous=true. This is a marker to see if we can reproduce. Notes:

1> this was pre-unifiedhighlighter.
2> unknown whether this is still an issue in more recent Solrs

I'll ask the OP to comment here with additional details.

Assigning to myself to track, I wont do any work on this for quite a while so anyone who wants to please take it.