If I use the SegmentMerger tool to merge many fetched content segments
(segments/<timestamp>/parse_text, etc.) into a single huge segment, do
I then create a performance problem when generating page summaries for
search hits? Are there contention or other issues reading these
fetched segments?

If there is a penalty, is there a recommended maximum size for content segments?

The cumulative size of all my content segments is in the neighborhood of 70GB.

Thanks,
Charlie