realloc() - frequency, conditions, or experiences about relocation?

In a recent thread realloc() was a substantial part of the discussion.
"Occasionally" the increased data storage will be relocated along
with the previously stored data. On huge data sets that might be a
performance factor. Is there any experience or are there any concrete
factors about the conditions when this relocation happens? - I could
imagine that it's no issue as long as you're in some kB buffer range,
but if, say, we're using realloc() to substantially increase buffers
often it might be an issue to consider. It would be good to get some
feeling about that internal.

Janis